table: Wire interposer consumer for memtable flush
table: Wire interposer consumer for memtable flush

From now on, memtable flush will use the strategy's interposer consumer
iff split_during_flush is enabled (disabled by default).
It has effect only for TWCS users as TWCS it's the only strategy that
goes on to implement this interposer consumer, which consists of
segregating data according to the window configuration.

Fixes #4617.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

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