Docs: typo on "frequent"

I replaced "high frequent terms" with "high frequency terms" and "low frequent terms" with "low frequency terms".

Alternatively, we could write, "highly frequent terms" and "minimally frequent terms" (or just "rare terms").

Closes #8962
Settings: Remove `mapping.date.round_ceil` setting for date math parsing

The setting `mapping.date.round_ceil` (and the undocumented setting
`index.mapping.date.parse_upper_inclusive`) affect how date ranges using
`lte` are parsed.  In #8556 the semantics of date rounding were
solidified, eliminating the need to have different parsing functions
whether the date is inclusive or exclusive.

This change removes these legacy settings and improves the tests
for the date math parser (now at 100% coverage!). It also removes the
unnecessary function `DateMathParser.parseTimeZone` for which
the existing `DateTimeZone.forID` handles all use cases.

Any user previously using these settings can refer to the changed
semantics and change their query accordingly. This is a breaking change
because even dates without datemath previously used the different
parsing functions depending on context.

closes #8598
closes #8889
Shutdown indices service last

We do wait for shards to be closed in IndicesService for 30 second.
Yet, if somebody holds on to a store reference ie. an open scroll request
the 30 seconds time-out and node shutdown takes very long. We should
release all other resources first before we shutdown IndicesService.

Closes #8940
Add toString() to IndexShardGateway