Remove locking in destructors

There's some basic architectural problems with the threading model Erizo uses, and I can see in an attempt to work around those there were locks added to the destructors of most of the major objects (WebRtcConnection, DtlsTransport, NiceConnection, etc.).  Unfortunately this just leads to deadlocks instead of crashes.  I've removed most of these and I'm going to run this code until I can work out some of the threading issues that are causing this. 

There's also a few other issues--NiceConnection queues a special packet to tell the transport to bugger off, but that packet was never being queued, and there's some locking that isn't as granular as it should be.