Try to make it a bit more obvious what's been going on in mock-request: add
timestamps to lines, and an identifier so that where we have two flushes in
parallel, we can see what's what.

Treat the waitTime passed into `flush` as a timeout rather than a
time-between-loops, so that we can pass in bigger times and not slow the tests
down too much.

Bump the timeout when waiting for /publicRooms and /initialSync in the joining
test.