perf: remove session pool (#751)

The underlying session pool has been completely removed from the driver, and
the driver always using a single multiplexed session for all operations. The
`min_sessions` and `max_sessions` connection properties are deprecated, and
setting a value for either of these has no effect.

Note that the driver was already using a single multiplexed session for all
operations by default before this change, but had a fallback option to use
a session pool. This fallback option is no longer available. This means that
the driver also does not need to create and maintain the sessions in the pool.

See https://docs.cloud.google.com/spanner/docs/sessions#multiplexed_sessions
for more information on multiplexed sessions in Spanner.

Author: olavloite