Remove unnecessary region locking from LockFile

Remove unnecessary region locking from LockFile

The lock file protocol relies on the atomic creation of a standardized
name in the parent directory of the file being updated.  Since the
creation is atomic, at most one thread in any process can succeed on
this creation, and all others will fail.  While the lock file exists,
that file is private to the thread that is writing it, and no others
will attempt to read or modify the file.

Consequently the use of the region level locks around the file are
unnecessary, and may actually reduce performance when using NFS, SMB,
or some other sort of remote filesystem that supports locking.

Change-Id: Ice312b6fb4fdf9d36c734c3624c6d0537903913b
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>