Feat: Back-fill null docs

[mster]

PROBLEM:
We know a query has N results from the queryList. However, in some cases Redis document hash keys may expire, dropping the document from the cache.

To the user, this means they get < N documents back, even though they expected N.

PROPOSAL:
Allow users to enable:

1. Back-filling of the cache after a returning only what the cache contained at the time (Could be zero documents).
2. Force back-filling of data before returning to the user, and then filling the cache afterwards.

(1) is fast*, while (2) is expensive yet dependable.

(*) the work still exists, but will occur on the next tick (has shown to slow down consecutive requests that result in cache misses).

[mster]

The time consideration is bordering on a non-issue as it occurs with only large document requests (>10k).

FireFerret is intended for front-end fetch apis which typically require small queries.