WL#10837: Deprecate query cache

Affects: Server-8.0   —   Status: Complete   —   Priority: Medium

Proposed changes:

1. Deprecation warnings in the parser:
* on `SQL_CACHE` and `SQL_NO_CACHE` syntax,
* on references to QC-related system variables.
2. Deprecation warnings in the command line help text of the `mysqld` program.
5.7
===

Add deprecation "no replacement" warnings to:

1. `SQL_CACHE` and `SQL_NO_CACHE` syntax in the parser,

2. any references to any QC-related system variables and help texts for their command-line counterparts:

* `have_query_cache`

* `query_cache_limit`

* `query_cache_min_res_unit`

* `query_cache_size`

* `query_cache_type`

* `query_cache_wlock_invalidate`

3. assignments to `ndb_cache_check_time` and its help text

4. `{RESET, FLUSH} QUERY CACHE` statements.


Note in the documentation, that QC-related variables (SHOW STATUS LIKE 'qc%') will be removed in the future release:

* `Qcache_free_blocks`

* `Qcache_free_memory`

* `Qcache_hits`

* `Qcache_inserts`

* `Qcache_lowmem_prunes`

* `Qcache_not_cached`

* `Qcache_queries_in_cache`

* `Qcache_total_blocks`


8.0
===

Since 8.0 still has (i.e. silently ignores) the `SQL_NO_CACHE` syntax and always returns *false* on the `have_query_cache` variable, maintain deprecation warnings for both like we do in 5.7 for a while.