Running an OxQL query holds database connections too long

[bnaecker]

Here's the main entrypoint to running an OxQL query:

omicron/oximeter/db/src/client/oxql.rs

Lines 334 to 343 in df990b0

	async fn run_oxql_query(
	&self,
	query_log: &Logger,
	handle: &mut Handle,
	query_id: Uuid,
	query: oxql::Query,
	total_rows_fetched: &mut u64,
	outer_predicates: Option<Filter>,
	outer_limit: Option<Limit>,
	) -> Result<OxqlResult, Error> {

That takes a Handle, a database connection claimed from the qorb connection pool in the client. We're holding that too long, most notably while running any in-memory table operations on the data. That's really not needed, and probably reduces overall throughput quite a bit. We should acquire new connections only as needed, when making a query against the database, and then drop them right after.