add a runner and test(s) for _reset_load_table_ai_counter

as seen in: https://github.com/cmu-delphi/delphi-epidata/blob/dev/src/acquisition/covidcast/database.py#L126

• also make change to ensure epimetric_load is empty first instead of just blindly clearing it

motivations for actual implementation reset process:

• "ALTER TABLE ... AUTO_INCREMENT = N can only change the auto-increment counter value to a value larger than the current maximum."
• "To reset the current auto-increment value ... You cannot reset the counter to a value less than or equal to the value that is currently in use"

extra credit:

• research (and implement?!?) if can we deny ability to TRUNCATE epimetric_load through permissions? (--> ie, remove drop permissions for epimetric_* family of tables to prevent resetting of autoincrement ids again)
• verify that we do not replicate epimetric_load table, though this is not a deal-breaker