Add locking to File interface

[trowski]

This adds three new methods to the File interface: lock, unlock, and getLockMode. As the names imply, these functions provide a non-blocking method obtaining and releasing a shared or exclusive lock on the file. The mode is provided by a new enum, LockMode with two cases: Shared and Exclusive. lock tries to obtain the lock in a loop using an exponential-backoff until a lock is obtained or the optional cancellation is cancelled. getLockMode returns the current lock mode if the file has been locked or null otherwise.

I've refactored FileMutex and KeyedFileMutex to use these two new async methods.

I think is a better option than #86, however it will require a new major version since the File interface has been modified. I don't consider this a blocker, as most components will be able to require 3.x or 4.x.

DO NOT merge into 3.x, we'll need make a 4.x branch if this is merged.

[trowski]

Updated to also add tryLock to the File interface which may be used to make a non-blocking attempt to lock the file, immediately returning true if the lock is obtained or false if not.