[#2843] Add test-case to show correct behavior of ByteBuf.refCnt() and ByteBuf.release(...)

Motivation:

We received a bug-report that the ByteBuf.refCnt() does sometimes not show the correct value when release() and refCnt() is called from different Threads.

Modifications:

Add test-case which shows that all is working like expected

Result:

Test-case added which shows everything is ok.
Reset interrupted flag in SingleThreadEventLoopTest. Fixes #2841

Motivation:

Once the interrupted flag in SingleThreadEventLoopB was set it was never reset.
That way the eventloop thread was always in an interrupted state and all calls
to Thread.sleep/LockSupport.parkNanos would fail.

Modifications:

Reset interrupted flag.

Result:

Number of GC runs when executing the test is down to zero (from 20 - 50).
Looking at the heap dump shows >10x decrease in memory consumption e.g.
the number of ForkJoinTasks is down to 25k from 350k previously. Similar
is true for BlockingQueue.Node objects.