XMPP connections leak memory

Over the last couple of weeks, we had to restart JBAC a couple of times because it was running out of heap memory and stuck doing full GCs. The last outage showed that the XMPP connections were sitting on around 1GB on memory. Despite the BambooSmackClient just used one connection, the other 531 were still referenced by the other 531 InBandBytestreamManager instances and thus couldn't be cleaned up by the garbage collector (see the size of 532 of the manager map).

I've also attached the memory usage statistics of CBAC which doesn't use the IM server integration (/admin/instantmessagingserver/viewInstantMessagingServer.action). It uses much less memory.