Reimplement MDEV-14275 Improving memory utilization for information schema

Postpone temp table instantiation until after setup_fields().

Replace all unused (not marked in read_set) columns in an I_S table
with CHAR(0). This can drastically reduce the footprint of a MEMORY
table (a TABLE_CATALOG alone is 1538 bytes per row).

This does not change the engine. If the table was decided to be Aria
(because of, say, blobs) then after optimization it'll stay Aria
even if all blobs were removed.

Note, when transforming table structure, share->blob_fields is preserved,
otherwise Aria might switch from DYNAMIC to STATIC row format and
expect a special field for a deleted mark, which create_tmp_tabe didn't
provide.

Note that optimizer was doing handler::info() (to know the number of rows)
before the temp table is populated. That didn't make much sense.
Now it's done before the table is even instantiated. Preserve the old
behavior and report 0 rows.

This reverts e2664ee8362

don't GRANT UPDATE ON mysql.global_priv TO mariadb.sys@localhost;

Classes that handle privilege tables (like Tables_priv_table)
could read some columns conditionally but they expect a certain
minimal number of colunms always to exist.

Add a check for a minimal required number of columns in privilege tables,
don't use a table that has fewer columns than required.

In commit 0e5a4ac2532c64a545796c787354dc41d61d0e62 (MDEV-15562)
we introduced a bogus debug assertion, demanding that dict_col_t::len
never exceed some maximum value. The intention was to match what
dict_index_add_col() is doing. But, the assignment field->fixed_len = 0
applies to dict_field_t::fixed_len, not dict_col_t::len!

An assertion

  `server_state_.rollback_mode() == wsrep::server_state::rm_async`

fired in before_command() when
- thread-handling was set to pool-of-threads and
- a BF abort happened between client session calls to
  wait_rollback_complete_and_acquire_ownership() and before_command().

This commit introduces a test case to reproduce the crash and
updates wsrep-lib submodule to fixed version.