LinuxContainer: AsyncMutex-ify LinuxContainer (#325)

This change moves LinuxContainer to use the new AsyncMutex type. This allows us to keep using our old state machine model, but lock around the entirety of operations. This also allows us to axe quite a few states that were intermediary states only there to protect against multiple threads calling (even if it was unlikely..) the same method at the same time (.creating, .starting, .stopping etc.).

The rationale is: Mutex is somewhat tricky to use as you can't (for good reason) do any async work inside the closure. This makes some of our work difficult as every single interaction with the guest is via an rpc, which is all async. There really isn't a worry about contention, because most container methods will only ever be invoked once (create, start, stop) so a big heavy lock isn't much of a deterrent.

Author: dcantah