in about a hundred of users of MY_BITMAP, only two were using its
built-in mutex, and only one of those two was actually needing it.

Remove the mutex from MY_BITMAP, remove all associated conditions
and checks in bitmap functions. Use an external LOCK_temp_pool
mutex and temp_pool_set_next/temp_pool_clear_bit acccessors.

Remove bitmap_init/bitmap_free, always use my_* versions.