Weak hash map usage in UnifiedInvokerHAProxy is not thread safe

txFailoverAuthorizations weak hashmap that is now also used for
maintaining transaction stickyness was created as an unsynchronized
WeakHashMap and this was causing issues.

It was not enough to synchronise on the tpc (key of map) whenever
we manipulated it because two threads, trying to store different tpcs
could end up trying to resize the map and as a result, key value
pairs could easily dissapear from the hash map.

This is extremely confusing because we thought the disappearance
was due to garbage collection.

From now on, we'll be using a Collections.synchronizedMap(new WeakHashMap());
instead. Thanks to Adrian for the help debugging this.