With the previous commit, there can be a scenario where batches that
had internal-updates-only can be rapidly introduced by the app, but
the persisted notifications on only the very last IndexSnapshot would
be fired. The persisted notifications on the in-between batches might
be missed.
The solution was to track the persisted notification channels at a
higher Scorch struct level, instead of tracking the persisted channels
at the IndexSnapshot and SegmentSnapshot levels.
Also, the persister double-check looping was simplified, which avoids
a race where an introducer might incorrectly not notify the persister.
This commit improves handling when an incoming batch has internal-data
updates only and no doc updates. In this case, a nil segment instead
of an empty segment instance is used in the segmentIntroduction. The
segmentIntroduction, that is, might now hold only internal-data
updates only.
To handle synchronous persistence, a new field that's a slice of
persisted notification channels is added to the IndexSnapshot struct,
which the persister goroutine will close as each IndexSnapshot is
persisted.
Also, as part of this change, instead of checking the unsafeBatch flag
in several places, we instead check for non-nil'ness of these
persisted chan's.
The cachedDocs preparation has to happen for all docs in the field,
not just on the currently requested docNum.
Also, as part of this commit, there's a loop optimization where we no
longer use bytes.Split() on the terms buffer, thus avoiding garbage
creation.
