clean up locking in FilePath*

[grandinj]

After looking at #2205 and concluding the FilePathSplit had deficient locking, I decided to look at the other file path implementations.

And it looks like only FilePathDisk is really OK (because it just uses the underlying FileChannel).

Now FilePath is supposed to be a FileChannel replica, so it should obey the same multi-threading semantics, which are document here:
https://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html
to be

File channels are safe for use by multiple concurrent threads. The close method may be invoked at any time, as specified by the Channel interface. Only one operation that involves the channel's position or can change its file's size may be in progress at any given time; attempts to initiate a second such operation while the first is still in progress will block until the first operation completes. Other operations, in particular those that take an explicit position, may proceed concurrently; whether they in fact do so is dependent upon the underlying implementation and is therefore unspecified.