mhnsw: inter-statement shared cache

* preserve the graph in memory between statements
* keep it in a TABLE_SHARE, available for concurrent searches
* nodes are generally read-only, walking the graph doesn't change them
* distance to target is cached, calculated only once
* SIMD-optimized bloom filter detects visited nodes
* nodes are stored in an array, not List, to better utilize bloom filter
* auto-adjusting heuristic to estimate the number of visited nodes
  (to configure the bloom filter)
* many threads can concurrently walk the graph. MEM_ROOT and Hash_set
  are protected with a mutex, but walking doesn't need them
* up to 8 threads can concurrently load nodes into the cache,
  nodes are partitioned into 8 mutexes (8 is chosen arbitrarily, might
  need tuning)
* concurrent editing is not supported though
* this is fine for MyISAM, TL_WRITE protects the TABLE_SHARE and the
  graph (note that TL_WRITE_CONCURRENT_INSERT is not allowed, because an
  INSERT into the main table means multiple UPDATEs in the graph)
* InnoDB uses secondary transaction-level caches linked in a list in
  in thd->ha_data via a fake handlerton
* on rollback the secondary cache is discarded, on commit nodes
  from the secondary cache are invalidated in the shared cache
  while it is exclusively locked
* on savepoint rollback both caches are flushed. this can be improved
  in the future with a row visibility callback
* graph size is controlled by @@mhnsw_cache_size, the cache is flushed
  when it reaches the threshold

sql/handler.h

@@ -559,6 +559,7 @@ enum legacy_db_type
 {
   /* note these numerical values are fixed and can *not* be changed */
   DB_TYPE_UNKNOWN=0,
+  DB_TYPE_HLINDEX_HELPER=6,
   DB_TYPE_HEAP=6,
   DB_TYPE_MYISAM=9,
   DB_TYPE_MRG_MYISAM=10,