cdc: introduce fields for keeping partition state
cdc: introduce fields for keeping partition state

Introduces data structures that will be used for keeping the current
state of processed rows: _clustering_row_states, and _static_row_state.

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