Java API: Make sure afterBulk is always called in BulkProcessor after beforeBulk

Moved BulkProcessor tests from BulkTests to newly added BulkProcessorTests class.
Strenghtened BulkProcessorTests by adding randomizations to existing tests and new tests for concurrent requests and expcetions.
Also made sure that afterBulk is called only once per request if concurrentRequests==0.

Closes #5038