Configure `max_logged_query_len` using a session variable

It would be useful to change the max logged query length at runtime without restarting the server. We currently provide this facility with the `@@dolt_log_level` system variable for log level. We coud do the same with a `@@dolt_max_logged_query_len` variable.

Start by tracing how the existing @@dolt_log_level session variable is defined and how the current maximum logged query length is read. Confirm the variable's runtime scope and identify the tests covering session variables and query logging. Done means @@dolt_max_logged_query_len changes the logging limit without restarting the server.

Written by the indexing model from the issue text.