Cache user/role mappings

This will avoid frequent db access. Mappings were read from database every time authorisation was required fro current user and authorisation was not cached. IN nexus authorisation idles in 2 minutes.
This is consistent with behaviour prior to migration of security from model to orient db, case when whole security configuration was kept in memory.

The change will also fix the failures we had due to interleaved db transactions between staging and security.