GerritStatusPush: Fix labels with gerrit >=2.6

Fixes #2550. Gerrit versions 2.6 and newer do not accept the
--verified option if the Verified label does not exist in All-Projects
in the gerrit configuration. Instead, we must give the --label option.
So, use --label for --verified and --code-review so we can work with
all gerrit installations.

While the --code-review option does still work, so does the --label
approach. It seems more likely that --label is more likely to work in
the future than --code-review, so we'll just go with the safer route.

In order to still support older gerrit installations (which do not
support the --label option), query the gerrit installation version
whenever we submit a review. Cache this version query for 10 minutes,
so we do not ask the gerrit server every single time for its version.
GerritStatusPush: Add per-buildset status updates

Currently GerritStatusPush will submit an update/review to gerrit
immediately when each builder in a buildset finishes building. It is
usually more useful to just get one update, containing information
about all of the builds in that buildset. So, allow GerritStatusPush
to do that, configurable via the new summaryCB constructor argument. A
default summaryCB is also provided: defaultSummaryCB.

If neither reviewCB nor summaryCB are explicitly provided in the
configuration, default to sending out the new "summary" reviews.
However, if reviewCB was explicitly provided, default to sending out
the old-style per-build reviews, and not sending out the new "summary"
reviews. This allows for older configurations to not see an unexpected
change in behavior, but completely new configurations are likely to
see the probably more useful "summary" reviews.

Since much of this functionality is already implemented in
MailNotifier, extract the relevant code into the new
BuildSetSummaryNotifierMixin, and make both MailNotifier and
GerritStatusPush use it. Document the new mixin class.

Also add tests for GerritStatusPush, to test the new behavior, and for
BuildSetSummaryNotifierMixin, to test that the mixin itself works
properly. Modify the unit tests for MailNotifier to accomodate the
refactoring.