event database will grow gradually, currently, even though we can
dump data to 3rd storage, the old data cannot be cleaned.
This patch adds a new option [database].event_time_to_live, and
create ttl index on event collection for MongoDB backend.
Also change previous [database].time_to_live to precise
[database].metering_time_to_live.
To enable test_bin, this patch also simply implements log://
partially implements blueprint event-database-ttl
DocImpact
Change-Id: I0d67cf212f37d3fa5555870a8e7cbb65af205d63
ef3d5612ab