Update ASIO wrapper constructors

... to only take BoostEnvironment and VMIdentifier (so the liveliness of the VM must be considered by the caller).
Also restores the behavior of creating a new TCPConnection in every call of startAsyncAccept().
Finally, constify a couple members of BoostEnvironment and BoostVM.
Potential flexibility, in particular the possibility of not loading from files on the file system (resources embedded in the executable, for example).