Slow query log per module

[joshua-spacetime]

#1071 captures slow queries through the SpacetimeDB server logs. Now we should give users the same insight with a slow query system table. They should be able to configure the 3 thresholds (subscriptions, ad hoc, incremental update) in the same way we can do now.

Something worth noting is that queries are executed in read-only transactions, but writing to a table requires a mutable transaction. This may require making read-only transactions upgradable. Basically the opposite direction of #1157.

[joshua-spacetime]

The schema of the slow query log

Name	Type	Description
query	string	text of the slow query
workload	enum	subscription, incremental update, ad hoc
duration	u64	duration of the query in milliseconds
identity	Identity	calling identity for this query

[cloutiertyler]

Upgrading to mutable is problematic because mutable queries require tracking reads, and you can't retroactively track reads that you've already read, so you always have to track reads for readers even if they are never upgraded.