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.