performance/quick-read: provide an invalidation based on ctime

Quick-read by default uses mtime to identify changes to file
data. However there are applications like rsync which explicitly set
mtime making it unreliable for the purpose of identifying change in
file content. Since ctime also changes when content of a file changes
and it cannot be set explicitly, it becomes suitable for identifying
staleness of cached data. This option makes quick-read to prefer ctime
over mtime to validate its cache. However, using ctime can result in
false positives as ctime changes with just attribute changes like
permission without changes to file data. So, use this option only when
mtime is not reliable.

credits to Kotresh Hiremath Ravishankar <khiremat@redhat.com> for
suggestion on using ctime instead of mtime.

Change-Id: Ib3ae39a3252b2876c8ffe81f471d02a87190e9b9
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Updates: bz#1591621

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