SHOW TABLES hive slowdown as of v443

[abridgett]

e.g for 70000 tables (ahem!) this was taking 4.5s but now takes 35s.

This is often used by client UIs in addition to typical usage and so an optimised version is probably justified.

The root cause is that in #21114 the optimised "get_all_tables call was dropped in favor of reusing "get_table_meta". (We don't want to cache to avoid dirty reads).

[ebyhr]

cc: @dain

[ebyhr]

Let's continue at #24164