db: Add configuration option for enabling SSTables 'mc' format.
db: Add configuration option for enabling SSTables 'mc' format.

This flag will only be used for testing purposes until Scylla 3.o
release and will be removed once SSTables 'mc' testing is completed.

Signed-off-by: Vladimir Krivopalov <dc9186a06078733915a6fcbab34e59120be2b484@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla