TTL mechanism support with expirer service

Aims to support a TTL mechanism for data entering Ceilometer
collector database.
TTL is very useful to configure when getting a lot of meters.

implement blueprint ttl-support
Change-Id: I9d98e8b833ad94fd79722dee7226e7f4f03aaa2e
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>