Index page row counts only for DBs with < 30 tables (10ms count limit per table)

[simonw]

Split out from #460.

If a database is mutable, calculating row counts gets expensive. I'm only going to calculate row counts for the index page if it has less than X tables (both hidden and non-hidden) AND each table can be counted in less than 10ms.

If any count takes longer than 10ms I'll cancel the counting entirely. We currently show an inaccurate count if this happens, which is just confusing.

[simonw]

I'm setting X to 30 because the fixtures database currently has 26 tables (22 visible, 4 hidden) and I want to display counts for it.

[simonw]

I'm going to be lazy and skip the unit test for this, because I don't currently have a neat way of mocking a SQL interrupted exception to simulate a query taking too long (at least for these counts).