Enable multithreaded tool calls by localizing use of MVar

[JBetz]

Resolves #11, though not in the same way I described in the issue. It still achieves the objective of enabling tool calls to be multi-threaded.

Difference is that instead of having the application decide on the concurrency mechanism for server state, it just uses MVar. This does make things a little awkward for the Stdio implementation which is using IORef. I'm not sure how to resolve that other than creating some sort of abstraction over MVar and IORef or by switching the entire Stdio implementation to using MVar. UPDATE: I switched it to MVar since it's more capable than IORef for concurrency, and you might want tool handlers to spin new threads that modify server state.

Some awkwardness is also removed as the current authenticated user is now separated from server state.

This is a breaking change.