features/bitrot: serialize versioning

Current signing interface (fsetxattr()) had couple of issues:

One, a signing request (by bitrot daemon) is denied if the version
against which an object is to be signed is unequal to the current
version of the object (cases where another subsequent modification
increments the version). Such request(s) are rejected with EINVAL
sent back to the signer resulting in a bunch of errors (in logs)
reported by bitrot daemon. Although, the object would be eventaully
signed with the version matching the current version, the "lagging"
request should be correctly handled.

Two, more than one signing request could race against each other
with the object getting signed with a version depending on which
request ended up last in the race. Although harmless to some extent,
such a case could end up marking the object's signature as stale
for infinity (if the object is *never* touched) thereby resulting
in scrubber skipping the object during verification.

This patch fixes these issues by ordering signing request(s) and
fixing version comparison checks at the time of signing.

Change-Id: I9fa83dfa3be664ba4db61d7f2edc408f4bde77dd
BUG: 1221938
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/10832
Tested-by: Gluster Build System <jenkins@build.gluster.com>

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