Closes quickwit-oss#896 - Facet reader related

Bugfix. Acquiring a facet reader on a segment that does not contain any
doc with this facet returns `None`.

Bumped version and edited changelog

Bumped version

Merge branch 'master' of github.com:tantivy-search/tantivy

Bump version

Bumped version for hotfix

Added README in tantivy-query-grammar

Hotfix quickwit-oss#681

Fixing the construction of the DeleteBitset.
Cherry picked from master.

Changing version to 0.10.3

SegmentUpdater.add_segment does not need to return true (quickwit-oss…

…#693)

Address quickwit-oss#656

Broke the reference loop to make sure that the watch_router can
be dropped, and the thread exits.