Expose fewer methods for tables

[roed314]

Ported from LMFDB's #3262:

Here are some possible things to put underscores in front of.

For db:

• can_read_write_knowls
• can_read_write_userdb
• cursor
• fetch_userpassword
• grant_* (maybe)
• is_alive
• is_read_only
• is_verifying
• log_db_change
• logger
• register_object

Note that the db namespace is cluttered with all the table names, so finding function names through tab completion is a bit annoying anyway.

For a table:

• has_id
• log_db_change
• logger