support SQL semantics for SELECT ... WHERE ... ORDER BY ... LIMIT

* switch from returning k nearest neighbors to returning
  as many as needed, in k-neighbor chunks, with increasing distance
* make search_layer() skips nodes that are closer than a threshold
* read_next keeps a search context - list of k found nodes,
  threshold, ctx, etc.
* when the list of found nodes is exhausted, it repeats the search
  starting from last found nodes and a threshold
* search context kepts ctx->refcount incremented, so ctx won't go away
* but commit_lock is unlocked between calls, so InnoDB can modify the table
* use ctx version to detect that, switch to MHNSW_Trx when it happens

bugfix:
* use the correct lock in ha_external_lock() for the graph table
* InnoDB didn't reset locks on ha_external_lock(F_UNLCK) and previous
  LOCK_X leaked into the next statement