AmazonTimestreamQuery client object leak

[rajeiyer]

[Issue Description]
CX has identified a memory leak (specifically, a thread leak) in the Timestream JDBC driver version 1.0.2.

There are three location in the driver that create an AmazonTimestreamQuery client object without subsequently calling its shutdown() method. Each new client object creates an AmazonTimestreamQueryEndpointCache which starts a thread to refresh the endpoint discovery cache. That thread will continue to run unless shutdown() is called.

Issue Details:

In each of the 3 locations where this occurs, a new AmazonTimestreamQuery object is created where instead the existing client object associated with the TimestreamConnection should probably be used. These locations are:

• TimestreamStatement constructor, line 82: object is created, but shutdown is not called on TimestreamStatement.close()
  -TimestreamStatement setQueryTimeout(), line 508: object is created, old object in member variable is discarded and shutdown is never called
• TimestreamConnection.isValid(), line 312-316: object is created and discarded within the method, shutdown is never called