added option to reset streaming connection in sampler

[ggs-jan]

In our test case, we create a new streaming connection for every thread loop, which is used only in that exact loop. In long running tests we want to prevent unused streaming connections from filling up the memory, so we need to make sure the connections get closed after every loop (the response pattern unfortunately is too unreliable here). As a solution, we added the option "Reset streaming connection" to the sampler to close any existing streaming connection under the given name and create a new one.

Additionally, we moved initialization and shutdown of the ExecutorService to the testStarted/testEnded methods, as not shutting down the ExecutorService properly resulted in the JVM not exiting after the test had finished.