Add some procedure to control number of threads plugin creates.

This has been released, reasoning behind our decision to remove the check can be found here:

if jenkins invocation requires merge check and this check is slow then plugin may create a lot of threads which suffocates the Bitbucket instance. Bitbucket itself runs these merge checks in single thread per node.
So options might be:

• use job scheduler which uses queue and bounded number of thread

• use separate job scheduler (with limited # of threads) for the tasks which require merge check

• I think in some cases we can analyze # of threads and skip merge checks or jenkins invocation at all (this can be done by special type of task queue)

final solution could be combination of above