Make Memory adapter threadsafe

[bkeepers]

Addresses #699 (comment)

This uses Concurrent::Hash (which just uses Hash on MRI since any C function is thread safe and Hash is implemented in C) as the backend source for the memory adapter and replaces any non-threadsafe uses (like ||=).

See update

[bkeepers]

Update: After experimenting Concurrent::Hash and several other classes from concurrent-ruby, I settled on just using a Mutex to wrap all adapter operations. It makes it a lot cleaner, and since this is all data in memory it should have minimal performance impact.