* fix: stop sending rpcs on deleted db

* fix: client should stop sending rpcs after database dropped

DatabaseClients should not continue to try to send RPCs to a database that has
been deleted. Instead, the session pool will keep track of whether a database
not found error has been returned for a database, and if so, will invalidate
itself. All subsequent calls for this database will return a DatabaseNotFoundException
without calling a RPC.

If a database is re-created, the user must create a new DatabaseClient with a new
session pool in order to resume usage of the database.

Fixes #16

* fix: remove double check on isValid

* fix: add wait to deleted db integration test

* fix: process review comments

* fix: update copyright year

* perf: close sessions async

Sessions should be closed using an async gRPC call in order
to speed up the closing of a large session pool. Instead of
using its own executor, which is limited to 8 worker threads,
to execute asynchronous delete session calls, the session pool
should use the asynchronous call option in gRPC. This allows a
larger number of asynchronous delete session calls to be executed
in parallel and speeds up closing a session pool with a large
number of sessions.

Testing against a real Cloud Spanner database with a session pool
containing 1,000 sessions shows the following performance for
closing the session pool:

Before (3 runs):
6603ms
8169ms
8367ms

After (3 runs):
1297ms
1710ms
1851ms

Fixes #19.

* fix: wait for test servers to terminate

* remove tracing for async call

* do not use directExecutor which could be a gRPC thread

* fix: return failed future instead of throwing exception

* fix: remove commented code

* updated versions.txt [ci skip]

* updated google-cloud-spanner-bom/pom.xml [ci skip]

* updated google-cloud-spanner/pom.xml [ci skip]

* updated grpc-google-cloud-spanner-admin-database-v1/pom.xml [ci skip]

* updated grpc-google-cloud-spanner-admin-instance-v1/pom.xml [ci skip]

* updated grpc-google-cloud-spanner-v1/pom.xml [ci skip]

* updated pom.xml [ci skip]

* updated proto-google-cloud-spanner-admin-database-v1/pom.xml [ci skip]

* updated proto-google-cloud-spanner-admin-instance-v1/pom.xml [ci skip]

* updated proto-google-cloud-spanner-v1/pom.xml [ci skip]

…nner (#37)

* test: add Commit Timestamp java integration tests for cloud spanner

* Use org.threeten.bp library instead of java.time for Java 7 compatibility

* test: Fix java comment documentation style to use @test annotation right before function name

* Fix formatting error in imports

This reverts commit ab25087.

* replace deprecated registration of the span names for zpages

* remove unused END_SPAN_OPTIONS

* fix test

* Revert "Revert "perf: close sessions async (#24)" (#43)"

This reverts commit 809ed88.

* Ignore compatibility check failure in internal interfaces.

asyncClose() was added to com.google.cloud.spanner.Session and
asyncDeleteSession() was added to
com.google.cloud.spanner.spi.v1.SpannerRpc in #24 which resulted in
binary compatibility test failures. This config allows us to ignore the
failure.

* Annotate SpannerRpc and Session classes as @internalapi.

Users shouldn't be implementing these interfaces as they're internal to
the client library implementation.

The ITClosedSessionTest could cause a flaky failure because it
deletes sessions on the server and then repeatedly executes queries
on the session until it returns a NOT_FOUND error. The actual deletion
could however happen exactly at the moment that the test query was
executed, which will cause Cloud Spanner to return a different error
message than the standard 'Session not found' error message.

Fixes #41

* updated google-cloud-spanner/pom.xml [ci skip]

* updated CHANGELOG.md [ci skip]

* updated grpc-google-cloud-spanner-admin-database-v1/pom.xml [ci skip]

* updated README.md [ci skip]

* updated grpc-google-cloud-spanner-admin-instance-v1/pom.xml [ci skip]

* updated versions.txt [ci skip]

* updated google-cloud-spanner-bom/pom.xml [ci skip]

* updated google-cloud-spanner/pom.xml [ci skip]

* updated grpc-google-cloud-spanner-v1/pom.xml [ci skip]

* updated pom.xml [ci skip]

* updated proto-google-cloud-spanner-admin-database-v1/pom.xml [ci skip]

* updated grpc-google-cloud-spanner-admin-database-v1/pom.xml [ci skip]

* updated proto-google-cloud-spanner-admin-instance-v1/pom.xml [ci skip]

* updated proto-google-cloud-spanner-v1/pom.xml [ci skip]

* updated grpc-google-cloud-spanner-admin-instance-v1/pom.xml [ci skip]

* updated grpc-google-cloud-spanner-v1/pom.xml [ci skip]

* updated pom.xml [ci skip]

* updated proto-google-cloud-spanner-admin-database-v1/pom.xml [ci skip]

* updated proto-google-cloud-spanner-admin-instance-v1/pom.xml [ci skip]

* updated proto-google-cloud-spanner-v1/pom.xml [ci skip]