Update ASIO wrapper constructors to only take BoostEnvironment and VMIdentifier

* Restore the behavior of creating a new TCPConnection
  in every call of startAsyncAccept().
Document a few exceptions for the calling thread
Constify a couple members of BoostEnvironment and BoostVM
Improve member order of BoostVM

* The environment is the "main context" of the VM.
* Also follows better the constructor arguments order.
Merge pull request #234 from eregon/boostvm_refs

Update ASIO wrapper constructors