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.
Indeed, but there is no other boot loader in the code base.
What was the intention and what could replace it?