Driver should support customizable timeout behavior

[battesonb]

The QLDB driver manages retryable errors on the consumer's behalf, however a custom session and transaction timeout may useful for those that want to respond quickly to failure. This can be replicated by dropping communication to the QLDB service using pf or iptables.

Additionally, it may be worth providing an execute timeout for transactions.

I have created a branch of a quick proposal which does not explicitly include custom retry behavior: https://github.com/awslabs/amazon-qldb-driver-java/tree/quickstart. It is currently not well-tested, but gets the idea across.