feat: expose reserveIds API

[athakor]

Fixes #53

Proposed Design Doc.

~/cc: @BenWhitehead

[codecov]

Codecov Report

Merging #56 into master will decrease coverage by 0.19%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master      #56     +/-   ##
===========================================
- Coverage     59.44%   59.25%   -0.2%     
+ Complexity      384      378      -6     
===========================================
  Files            46       46             
  Lines          2286     2280      -6     
  Branches        251      250      -1     
===========================================
- Hits           1359     1351      -8     
- Misses          820      822      +2     
  Partials        107      107

Impacted Files	Coverage Δ	Complexity Δ
...java/com/google/cloud/datastore/DatastoreImpl.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...oogle/cloud/datastore/spi/v1/HttpDatastoreRpc.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...n/java/com/google/cloud/datastore/PathElement.java	79.06% <0%> (-0.48%)	23% <0%> (-3%)
.../src/main/java/com/google/cloud/datastore/Key.java	79.1% <0%> (-0.31%)	16% <0%> (ø)
...java/com/google/cloud/datastore/IncompleteKey.java	87.8% <0%> (-0.3%)	9% <0%> (ø)
.../main/java/com/google/cloud/datastore/BaseKey.java	89.39% <0%> (-0.16%)	14% <0%> (ø)
...va/com/google/cloud/datastore/StructuredQuery.java	71.38% <0%> (-0.08%)	23% <0%> (ø)
...a/com/google/cloud/datastore/QueryResultsImpl.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...in/java/com/google/cloud/datastore/KeyFactory.java	100% <0%> (ø)	7% <0%> (-3%)	⬇️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2096777...be9dc01. Read the comment docs.

[athakor]

@BenWhitehead PTAL

[BenWhitehead]

Why do we need the instanceof check and Key -> pb -> IncompleteKey -> Key dance here?

[athakor]

thanks for pointing out this, it's look like unused, I have already simplified this.

[athakor]

@BenWhitehead all the comment has been addressed. PTAL

[BenWhitehead]

One small change, after that this will be ready to merge.

[BenWhitehead]

Thanks for the feature @athakor!