multishard_combining_reader: fix handling of non-strictly monotonous positions
multishard_combining_reader: fix handling of non-strictly monotonous positions

The shard readers under a multishard reader are paused after every
operation executed on them. When paused they can be evicted at any time.
When this happens, they will be re-created lazily on the next
operation, with a start position such that they continue reading from
where the evicted reader left off. This start position is determined
from the last fragment seen by the previous reader. When this position
is clustering position, the reader will be recreated such that it reads
the clustering range (from the half-read partition): (last-ckey, +inf).
This can cause problems if the last fragment seen by the evicted reader
was a range-tombstone. Range tombstones can share the same clustering
position with other range tombstones and potentially one clustering row.
This means that when the reader is recreated, it will start from the
next clustering position, ignoring any unread fragments that share the
same position as the last seen range tombstone.
To fix, ensure that on each fill-buffer call, the buffer contains all
fragments for the last position. To this end, when the last fragment in
the buffer is a range tombstone (with pos x), we continue reading until
we see a fragment with a position y that is greater. This way it is
ensured that we have seen all fragments for pos x and it is safe to
resume the read, starting from after position x.

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla