Opt-in is possible by passing an engaged `compaction_time`
(gc_clock::time_point) to the method. When this new parameter is
disengaged, no compaction happens.
Note that there is a global override, via the
enable_compacting_data_for_streaming_and_repair config item, which can
force-disable this compaction.
Compaction done on the output of the streaming reader does *not*
garbage-collect tombstones!
All call-sites are adjusted (the new parameter is not defaulted), but
none opt in yet. This will be done in separate commit per user.
42b0dd5558