Log descriptor-download bootstrapping messages less verbosely

This is a fix for 9963.  I say this is a feature, but if it's a
bugfix, it's a bugfix on 0.2.4.18-rc.

Old behavior:

    Mar 27 11:02:19.000 [notice] Bootstrapped 50%: Loading relay descriptors.
    Mar 27 11:02:20.000 [notice] Bootstrapped 51%: Loading relay descriptors.
    Mar 27 11:02:20.000 [notice] Bootstrapped 52%: Loading relay descriptors.
    ... [Many lines omitted] ...
    Mar 27 11:02:29.000 [notice] Bootstrapped 78%: Loading relay descriptors.
    Mar 27 11:02:33.000 [notice] We now have enough directory information to build circuits.

New behavior:

    Mar 27 11:16:17.000 [notice] Bootstrapped 50%: Loading relay descriptors
    Mar 27 11:16:19.000 [notice] Bootstrapped 55%: Loading relay descriptors
    Mar 27 11:16:21.000 [notice] Bootstrapped 60%: Loading relay descriptors
    Mar 27 11:16:21.000 [notice] Bootstrapped 65%: Loading relay descriptors
    Mar 27 11:16:21.000 [notice] Bootstrapped 70%: Loading relay descriptors
    Mar 27 11:16:21.000 [notice] Bootstrapped 75%: Loading relay descriptors
    Mar 27 11:16:21.000 [notice] We now have enough directory information to build circuits.
Merge remote-tracking branch 'public/bug9963_v2_024'