Associated TTLs with Column Families instead of Granularity.

* This only caused a problem with the Account interface in TtlCache, which probably needs to be rethought anyway.
* Introduce MetricColumnFamily that contains a default TTL.
* Get all the TTL stuff out of Granularity.  woo.
Merge pull request #142 from gdusbabek/associated_default_ttl_with_CF

Associate TTLs with Column Families instead of Granularity.