resetStreamer() does not close the underlying Entity Manager

[julgus]

Describe the bug
Whenever resetting a Streamer using jpaStreamer.resetStreamer() the underlying Entity Manager is left open. This can cause memory leaks, see #147.

Expected behavior
The Entity Manager associated with the removed Streamer should be closed, as we are no longer holding a reference to it. This also prevents JPAStreamer from properly closing these Entity Managers upon jpaStreamer.close() as they are no longer in the cache, but are still open.

Actual behavior
Any Streamer that we cache will create an Entity Manager that stays open if the Streamer is removed from the cache through a call to jpaStreamer.resetStreamer().

JPAStreamer version
e.g. JPAStreamer 3.0.1

[julgus]

Following discussions in #147, we decided to remove the cache altogether. Therefore resetStreamer() will be deprecated from 3.0.2.