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

2. hard-code ef_construction=10, best by test

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

4. rename hnsw_ef_search to mhnsw_min_limit and add a help text