Provide a slow query log

[fulghum]

MySQL's slow query log is really helpful to identify queries that are taking a long time to execute. Having something similar could help Dolt customers identify slow queries that are using up many resources on the DB host.

After first providing an opt-in slow query log file, providing a table function to access the log data without having to log into the host could be useful for customers.

Supporting the Slow_queries status variable would also be helpful for operational monitoring. Tools like Datadog use this status variable, as well as the slow query log itself, to provide operational dashboards.