Consolidate accounts DB index and slot-to-bankhash pebble DBs and use a batch for slot's account index updates

[palmerlao]

This didn't affect the space used by the mithril_db folder (46G in both cases) and the slot replay duration changes seem negligible or slightly improved:

mithril-baseline.log:
  n=998 mean=0.2476 p10=0.1600 p25=0.1950 p50=0.2530 p75=0.2990 p90=0.3310
mithril-0555cf0.log:
  n=998 mean=0.2488 p10=0.1607 p25=0.1940 p50=0.2530 p75=0.3030 p90=0.3343
mithril-57a9ee8.log:
  n=998 mean=0.2455 p10=0.1600 p25=0.1913 p50=0.2510 p75=0.3000 p90=0.3300