NEXUS-5472: Kill pool if threads not end cleanly

Added shutdownNow call used in case of needed, when
on shutdown the running threads cannot be canceled.

In some cases, like remote HTTP request being "stuck",
like there was case on Bamboo and Codehaus snapshots
repo, where request times out (30s) or for some reason
takes 20s to execute, this might help if this
happens during shutdown (which is often in our ITs)