1. introduce alpha. the value of 1.1 is optimal, so hard-code it.

2. define ef and efConstruction in terms of limit and M, that is,
   ef = ef_limit_multiplier * limit
   efConstruction = ef_construction_multiplier * M

3. ef_construction_multiplier=4 is almost always optimal, so
   hard-code it too

4. rename hnsw_max_connection_per_layer to mhnsw_max_edges_per_node
   (max_connection is rather ambiguous in MariaDB) and add a help text

5. rename hnsw_ef_search to mhnsw_limit_multiplier and add a help text