We know that sstable_run is supposed to contain disjoint files only,
but this assumption can temporarily break when switching strategies
as TWCS, for example, can incorrectly pick the same run id for
sstables in different windows during segregation. So when switching
from TWCS to ICS, it could happen a sstable_run won't contain disjoint
files. We should definitely fix TWCS and any other strategy doing
that, but sstable_run should have disjointness as actual invariant,
not be relaxed on it. Otherwise, we cannot build readers on this
assumption, so more complicated logic have to be added to merge
overlapping files.
After this patch, sstable_run will reject insertion of a file that
will cause the invariant to break, so caller will have to check
that and push that file into a different sstable run.
Closes#11116
0796b8c97a