Report bootstrapping progress correctly when downloading microdescs

We had updated our "do we have enough microdescs to begin building
circuits?" logic most recently in 0.2.4.10-alpha (see bug 5956), but we
left the bootstrap status event logic at "how far through getting 1/4
of them are we?"

Fixes bug 9958; bugfix on 0.2.2.36, which is where they diverged (see
bug 5343).