Handle write functions and advisory locks

[levkk]

Description

• Handle functions that write and require a primary. feat: side-effect SELECT #158 SELECT pg_advisory_lock #134 Description of how this works below.
• Fix bug in session cleanup (that feature wasn't used, so that's why it didn't hit before).
• Add more go/pgx tests.

Functions that write

There are a couple types of Postgres functions that write:

• Sequence manipulation, e.g. nextval. Simple to handle: send to primary.
• Advisory locks. These are more tricky, because they create a session-level state that we can't easily track. To make them work, the client connection that creates a lock will be tied to the server connection until the client disconnects. This effectively turns the pooler into a session mode, just for that client.