kl sstable reader: replace states for ending with one state, simplify non_consuming
kl sstable reader: replace states for ending with one state, simplify non_consuming

After removing the switch, the only use for states in the sstable reader
are methods non_consuming() and verify_end_state().

The non_consuming() method is only used after assuring that
!primitive_consumer::active() (in continuous_data_consumer::process())
so we don't need states where primitive_consumer::active() for this
method, and is actually all of them.

We don't differentiate between ATOM_START and ATOM_START_2 in
verify_end_state(), so we can just merge them into one.

While we need tho remember times when we enter states used in verify_end_state(),
we also need to remember when we exit them. For that reason we introduce a new
state "NOT_CLOSING", that fails all comparisons in verify_end_state(), and
replaces all states that aren't used in verify_end_state()

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