Creates stay fast with a thousand sandboxes

A create now costs the same beside a thousand of your account's sandboxes as beside none: what it checks (sandboxes at once, quotas, a server's room) is kept as a running total instead of counted again at each create. On a test database with one account on one server, a create's database work beside 1,000 sandboxes went from 11 to 85 ms to 6.3 to 9.7 ms, and 100 creates over 8 connections from 6 to 11.5 s to 0.5 to 0.6 s. Every limit holds exactly as before.