Some tests take quite some time during exit, which I think causes
some appveyor fails like this:

    https://ci.appveyor.com/project/ethereum/go-ethereum/builds/39511210/job/xhom84eg2e4uulq3

One of the things that seem to take time during exit is waiting
(up to 100ms) for the syncbloom to close. This PR changes it to use
a channel, instead of looping with a 100ms wait.

This also includes some unrelated changes improving the reliability of
eth/fetcher tests, which fail a lot because they are time-dependent.