Concurrent processing of events in hybrid MP+MT workers

Modified AthenaMPTools/SharedHiveEvtQueueConsumer so that it can process events concurrently. In order to achieve that this tool is now using the AthenaMtesEventLoopMgr as the event processor, instead of the previously used AthenaHiveEventLoopMgr.

Also introduced a helper abstract interface AthenaKernel/IHybridProcessorHelper to allow the SharedHiveEvtQueueConsumer tool to call certain methods of the AthenaMtesEventLoopMgr.

Please note:

1. For the sake of simplicity in this version I have explicitly disabled support for shared I/O - both Reader and Writer - in hybrid MP+MT jobs. The shared I/O can be reintroduced later, once we are happy with the current functionality.
2. I have also disabled support for running pileup digitization, which cannot run in AthenaMT these days anyway.

CC @amete @gemmeren @elmsheus who have previously expressed some interest in hybrid MP+MT.