support converting null TTLs to default TTL relative to write timestamp

[tarzanek]

before merging please add a disclaimer that for safety reasons such a migration where we tamper with writetimestamps and TTLs should be always done into an empty table

[tarzanek]

this merge is delayed, this approach uncovered some bugs in scylla, so merge will happen once Scylla has all the patches to support this injection

[tarzanek]

all officially supported branches have now fixes, we should revisit this and merge