feat: rollback a connection returned back to pool (googleapis#193)

* feat: drop read_only on a connection returned back to a pool

* feat: rollback a connection returned back to pool

Co-authored-by: Vikash Singh <3116482+vi3k6i5@users.noreply.github.com>

Author: Ilya Gurov

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -48,6 +48,7 @@
 @listens_for(Pool, "reset")
 def reset_connection(dbapi_conn, connection_record):
     """An event of returning a connection back to a pool."""
+    dbapi_conn.connection.rollback()
     if getattr(dbapi_conn.connection, "staleness", None) is not None:
         dbapi_conn.connection.staleness = None