features/bit-rot: Implementation of bit-rot xlator

This is the "Signer" -- responsible for signing files with their
checksums upon last file descriptor close (last release()).
The event notification facility provided by the changelog xlator
is made use of.

Moreover, checksums are as of now SHA256 hash of the object data
and is the only available hash at this point of time. Therefore,
there is no special "what hash to use" type check, although it's
does not take much to add various hashing algorithms to sign
objects with. Signatures are stored in extended attributes of the
objects along with the the type of hashing used to calculate the
signature. This makes thing future proof when other hash types
are added. The signature  infrastructure is provided by bitrot
stub: a little piece of code that sits over the POSIX xlator
providing interfaces to "get or set" objects signature and it's
staleness.

Since objects are signed upon receiving release() notification,
pre-existing data which are "never" modified would never be
signed. To counter this, an initial crawler thread is spawned
The crawler scans the entire brick for objects that are unsigned
or "missed" signing due to the server going offline (node reboots,
crashes, etc..) and triggers an explicit sign. This would also
sign objects when bit-rot is enabled for a volume and/or after
upgrade.

Change-Id: I1d9a98bee6cad1c39c35c53c8fb0fc4bad2bf67b
BUG: 1170075
Original-Author: Raghavendra Bhat <raghavendra@redhat.com>
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/9711
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>

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