raft: replication test: add dummy command support
raft: replication test: add dummy command support

Use a special value as dummy entry to be ignored when seen in state
machine input.

Ignore dummy entries for count.

Signed-off-by: Alejo Sanchez <004be06a28e38b437b038f9b5c0b9f5758b792e9@scylladb.com>

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