Pool pithos backends using ObjectPool

Use ObjectPool functionality from synnefo.lib.pool
to pool backend objects.

This reduces the per-request overhead significantly,
since each backend object is only initialized once,
and connections to the DB are persistent.

Also bump snf-common dependency to reflect need for
updated snf-common with ObjectPool support.

Signed-off-by: Georgios D. Tsoukalas <gtsouk@cslab.ece.ntua.gr>
Signed-off-by: Vangelis Koukis <vkoukis@grnet.gr>