perf: remove session pool (GoogleCloudPlatform#4124)

This version of PGAdapter uses a version of Spanner Java client library
that has removed the internal session pool altogether. This means that:
1. All operations are executed on a single multiplexed session.
   A multiplexed session can execute any number of queries or transactions
   in parallel.
2. All settings for the session pool (e.g. MinSessions and MaxSessions)
   have been deprecated. Setting them is still allowed, but has no effect.

Removing the session pool means that PGAdapter no longer creates 100 sessions
when it is started, and also no longer needs to keep these sessions alive in
the background.

Author: olavloite