TimeoutHashMap is not threadsafe

[trumpetinc]

Right now, I'm pretty sure that the TimeoutHashMap implementation is not threadsafe. This places a large burden on DefaultRPCSender to manage synchronization. Otherwise, two calls to put(...) could corrupt the underlying data structure. With the current timeout thread running, there is a race condition that could get really ugly under load.

I have committed load test demonstrates one of the issues within a few seconds (I believe that there are other concurrency issues as well, but this hits an easy one) - see https://github.com/ghetolay/jwamp/blob/master/jwamp-core/src/test/java/com/github/ghetolay/jwamp/utils/TimeoutHashMapLoadTest.java

The fix for this particular race is to have TimeoutHashMap extend from ConcurrentHashMap - but I would prefer to consider a different design for TimeoutHashMap (I will open another issue on that).