MDEV-20001 Potential dangerous regression: INSERT INTO >=100 rows fail for myisam table with HASH indexes

Problem:-

So the issue is when we do bulk insert with rows
> MI_MIN_ROWS_TO_DISABLE_INDEXES(100) , We try to disable the indexes to
speedup insert. But current logic also disables the long unique indexes.

Solution:- In ha_myisam::start_bulk_insert if we find long hash index
(HA_KEY_ALG_LONG_HASH) we will not disable the index.

This commit also refactors the mi_disable_indexes_for_rebuild function,
Since this is function is called at only one place, it is inlined into
start_bulk_insert

mi_clear_key_active is added into myisamdef.h because now it is also used
in ha_myisam.cc file.

(Same is done for Aria Storage engine)