dtss/rocksdb: fix for reading across unfilled fragments

Overview

An issue found when writing two fragments to dtss/rocksdb, and then reading back the resulting ts.

To illustrate:

if the rocksdb fragement size was n, and writing a ts fragment of n-3, then write another fragment starting at position n.

Then reading back the entire ts, the gap (n-3.. n) was UMR copied, leaving stray data into the observed ts.