[MDEV-5471] Miscellaneous table cache improvements - JIRA

Details

Add status variable for expensive table cache evictions (from tc_add_table()).
Add status variable for cheap table cache evictions (from tc_release_table()).
Expensive eviction: evict 10 (or 20, or 100, or 10%) tables at once.
~~Use recently implemented Atomic class for all atomic variables.~~ (Atomic class was declined)
Make sure LOCK_open is on a separate cache line (at least until it is used by tc_release_table()). It was seen to cause false sharing with tc_count at least.
Add an inline getter method TABLE::in_use() that in debug builds will assert: DBUG_ASSERT(table->in_use == current_thd || (table->s->flushed && table->in_use))
ha_table_exists() - instead of calling tdc_lock_share()/tdc_unlock_share() do hash lookup.

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