Add timeout-constrained version of execute on RiakClient

[alexmoore]

This supersedes PR #610 (CLIENTS-922), by adding tests to the original PR.

[alexmoore]

Special thanks to @bwittwer for the original PR!

[lukebakken]

@alexmoore please to be fixing your builds :-P

[WARNING] /home/buildbot/masters/riak/java-client-test/build/src/test/java/com/basho/riak/client/core/RiakClientTest.java:[53,90] unchecked cast
  required: com.basho.riak.client.core.RiakFuture<java.lang.String,java.lang.String>
  found:    com.basho.riak.client.core.RiakFuture

[lukebakken]

What happens when you execute a long-running Riak command, but specify a short timeout? Does the client disregard the value when returned by Riak?

[alexmoore]

Yep, the command's execution will throw a TimeoutException(), and unblock the user thread. The operation will continue to process on Riak && the client's backend/netty threads until it completes in some fashion.

[lukebakken]

OK I just want to make sure that the data returned from Riak is discarded and not used somehow by a different command.

[lukebakken]

Other than my concern, 👍

[alexmoore]

Nope, it should be discarded once the operation is complete, and the resources returned to the pool. It'll keep running in the background, but we'd need to add operation cancellation in to fix that (another issue).