Reduces coupling. User's should not rely on the fact that it's an
std::map<>. It also allows us to extend row's interface with
domain-specific methods, which are a lot easier to discover than free
functions.
Origin does that, so should we. Both ttl and expiry time are stored in
sstables. The value of ttl seems to be used to calculate the read
digest (expiry is not used for that).
The API for creating atomic_cells changed a bit.
To create a non-expiring cell:
atomic_cell::make_live(timestamp, value);
To create an expiring cell:
atomic_cell::make_live(timestamp, value, expiry, ttl);
or:
// Expiry is calculated based on current clock reading
atomic_cell::make_live(timestamp, value, ttl_optional);
Ensure that read-side accessors are const. This is important in preparation
for multiple memtables (and later, sstables) since a read-side
mutation_partition may be a temporary object coming from multiple memtables
(and sstables) while a write-side mutation_partition is guaranteed to belong
to a single memtable (and thus, not be temporary).
Since writers will want non-const mutation_partitions to write to, they won't
be able to use the read-side accessors by accident.
Partitions should be ordered using Origin's ordering, which is first
by token, then by Origin's representation of the key. That is the
natural ordering of decorated_key.
This also changes mutation class to hold decorated_key, to avoid
decoration overhead at different layers.
