BUG-994: introduce object-cache into distribution

This is the prerequisite for
https://git.opendaylight.org/gerrit/#/c/6926/, which introduces
dependency on object-cache-api. We also add implementation based on
Guava's Cache.

Change-Id: I798750bf95da540ee1c95c4ac18977729322a291
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-994: introduce object-cache into distribution"