Add optional support for write locks

These write locks prevent accidental misuse of `Synchronizer`s by
establishing an advisory lock on the state file using the `flock`
system call. Correctly configured writers will check this lock to
prevent duplicate writers.

By default, write locks are disabled and carry no additional cost
through use of generics. Unused functionality is optimized out by
the compiler during monomorphization.

Write locking is only supported on Unix-like platforms due to use
of the `flock` API. Other locking mechanisms and lock modes could
be supported in the future.