cached memory only indexes

Was talking to Ritesh M. and he was describing so-called "real time indexes" from solr (or lack thereof).

The thought I had was to consider yet another KV storage engine for bleve, but memory only, with cache-like semantics (throws away entries) to try to have limited memory bound.

Probably also needs to be matched up with a datasource (or Feed) that knows how to favor getting the latest, newest data onwards (as opposed to starting from zero with a full backfill). And/or, the (imaginary) Feed knows how to ask for only the data that the datasource (like KV engine) has in memory already.

The issue names no files, tests, or concrete entry points. Start by reviewing Bleve's existing KV-storage and datasource/Feed abstractions, then clarify the memory-bound and newest-data requirements with maintainers; the issue currently provides no agreed definition of done.

Written by the indexing model from the issue text.