This will essentially make dict_sys.latch a mutex
(it is only acquired in exclusive mode).

The subsequent commit will merge dict_sys.mutex into dict_sys.latch
and reintroduce dict_sys.freeze() for those cases where we currently
acquire only dict_sys.latch but not dict_sys.mutex. The case where
both are acquired will be mapped to dict_sys.lock().

i_s_sys_tables_fill_table_stats(): Invoke dict_sys.prevent_eviction()
and the new function dict_sys.allow_eviction() to avoid table eviction
while a row in INFORMATION_SCHEMA.INNODB_SYS_TABLESTATS is being
produced.

Reviewed by: Thirunarayanan Balathandayuthapani