Support some notion of exclusive terminal access

[alexrp]

For input processing scenarios, one might either subscribe to TerminalReader.InputRead or call TerminalReader.ReadBuffer directly. The latter will have higher overhead, especially if the event handler needs to fire off a task to prevent blocking. The former will have lower overhead but will essentially take over standard input. (Both are valid options.)

For the latter case, we need a way to formalize the exclusive access to the terminal such that anyone else attempting to access it will get an exception.

[alexrp]

ReaderWriteLockSlim not working in async methods is throwing a wrench in my approach here...