RFR: Add Config based TTL support for BF. 

This is step 0 of TTL work. 

Some interesting discussions happened here:
https://github.com/lakshmi-kannan/blueflood/pull/11

Email thread on TTL:
https://groups.google.com/forum/#!topic/blueflood-discuss/MN9CuGbwGOM

The subsequent step is to use the ConfigTtlProvider in ingest and rollup code so we can deprecate the existing InternalAPI TTL path. I'll work on a different PR for that.