Function mempool_scan_budget 

fn mempool_scan_budget() -> Duration

How long one pass may spend fetching mempool transactions before it publishes what it has and starts the next pass.

Fetches are sequential, so a cold cache over a large mempool would otherwise hold back the confirmations already gathered from the block scan until every transaction had been fetched. Kept short under test so that a test can observe a pass running out of budget without a large mempool.