Problem was that we could take page latches on different
order than wat is entitled with SX-lock. To follow the
latching order defined in WL#6326, acquire index->lock X-latch.
This entitles us to acquire page latches in any order for the index.
btr0btr.cc
Document latch rules before and after MariaDB 10.2.2
sync0rw.cc
Document latch compatibility rules better.
btr_defragment_merge_pages
Fix parameter value.
btr_defragment_thread
Acquire X-lock to dict_index_t::lock before restoring
cursor position and continuing defragmentation.
ha_innobase::optimize
Restore defragment feature.
Testing
Add GIS-index and FT-index to table being defragmented.
Defragmentation is not done to GIS-indexes and FT auxiliary
tables.
