Expose timestamp of the oldest key in history to kv_engine from Magma

Magma keeps track of the timestamp of the oldest key in history per vbucket/kvstore. It is the CAS of low key of the first SSTable in history converted to seconds(CAS is in microseconds).

Atm it is only printed in the stats.log during a cbcollect vis the MagmaStats object and we do not have history of the stat via prometheus. We need to keep track of their stat over time and expose it to kv_engine.