In CQL a row is considered as present if its row marker is live or it
has any cells live. The 'insert' statement creates a row
marker. Internally Origin handles that by inserting a special cell
whose name shares the prefix with other cells in that row.
One consequence of this way of things is that when we query a column
slice from sstables we will have to read the whole CQL row, even if
not all columns are queried. We won't have to include the data, but we
will need liveness information in order to commute it with other
mutations, so that we can finally determine if the row is live or not.
Holding keys and their prefixes as "bytes" is error prone. It's easy
to mix them up (or use wrong types). This change adds wrappers for
keys with accessors which are meant to make misuses as difficult as
possible.
Prefix and full keys are now distinguished. Places which assumed that
the representation is the same (it currently is) were changed not to
do so. This will allow us to introduce more compact storage for non-prefix
keys.
