Using build percentage is sub-optimal since many events can reset the
percentage which can result in additional requests being merged into the
staging. A better metric is the time since the first request was added to
the staging (the age of the staging). Unfortunately, this is not trivial to
determine especially given that the original request may be superseded or
unstaged entirely. As such the datetime at which the staging was activated
is stored in the staging pseudometa. A max age is then used to allow for
merging up until that point.