Modify the methods which calculate the default gc mode as well as that
which validates whether repair-mode can be used at all, so both accepts
use of repair-mode on RF=1 tables.
This de-facto changes the default tombstone-gc to repair-mode for all
tables. Documentation is updated accordingly.
Some tests need adjusting:
* cqlpy/test_select_from_mutation_fragments.py: disable GC for some test
cases because this patch makes tombstones they write subject to GC
when using defaults.
* test/cluster/test_mv.py::test_mv_tombstone_gc_not_inherited used
repair-mode as a non-default for the base table and expected the MV to
revert to default. Another mode has to be used as the non-default
(immediate).
* test/cqlpy/test_tools.py::test_scylla_sstable_dump_schema: don't
compare tombstone_gc schema extension when comparing dumped schema vs.
original. The tool's schema loader doesn't have access to the keyspace
definition so it will come up with different defaults for
tombstone-gc.
* test/boost/row_cache_test.cc::test_populating_cache_with_expired_and_nonexpired_tombstones
sets tombstone expiry assuming the tombstone-gc timeout-mode default.
Change the CREATE TABLE statement to set the expected mode.
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