DRBD: activity_log

Within DRBD the activity log is used to track extents (4MB each) in which IO
happens (or happened recently). It is based on the LRU cache. Each change of
the activity log causes a meta data update (single sector write).  The size
of the activity log is configured by the user, and is a tradeoff between
minimizing updates to the meta data and the resync time after the crash of a
primary node.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>