MB-20852 [4/N]: Use named struct when moving cursors between checkpoints
Instead of an anonymous std::pair (which is hard to follow what the
two fields are), use a named struct `CursorPosition` and related
CursorIdToPositionMap map when recording cursor positions to move
between checkpoints.
Change-Id: I39d14dbdae34cd20fe880e23b95d01c5fbfdac91
Reviewed-on: http://review.couchbase.org/69014
Reviewed-by: Jim Walker <jim@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>