rgw/cache: implementation for organizing data in ssd cache

rgw/cache: implementation for organizing data in ssd cache
into bucket directory, object directory and then versions
representing file names.

This commit for extracts bucket id, object name, version,
offset, and length from input key and organizes data into
directories - with bucket id, object name being used to
create directories and filenames within them using version
and offset and length.

Data can be restored back to in memory LFUDA and dirty object
data structure once rgw is restarted.

Updating unit tests.
test/rgw: Correct RedisDriver keys and add checks for cache blocks after
write

Signed-off-by: Pritha Srivastava <prsrivas@redhat.com>
Signed-off-by: Samarah <samarah.uriarte@ibm.com>