To make the counter increment polymorphic between
attributes that use Redis counter and legacy Rails counter,
the increment implementation is extracted into two counter
classes: Gitlab::Counters::BufferedCounter and
Gitlab::Counters::LegacyCounter.

Redis implementation is extracted from the concern
into the BufferedCounter class.

LegacyCounter wraps around existing ActiveRecord

Operations on ProjectStatistics such as
refreshing storage size and scheduling ScheduleAggregationWorker
are performed in an after_commit_callback.