We expect each replica to stop at exactly the same position when the
digests match. Soon however, if replicas have a lot of tombstones, some
may stop earlier then the others. As long as all digests match, this is
fine but we need to make sure we continue from the smallest such
positions on the next page.
A simple struct containing a full position, including a partition key
and a position in partition. Two variants are introduced: an owning
version and a view. This is to replace all the ad-hoc structures
introduced for the same purpose: std::pair() and std::tuple() of
partition key and clustering key, and other similar small structs
scattered around the code.
This patch does not replace any of the above mentioned construcs with
the new full_position, it merely introduces it to enable incremental
standardization.
