write() will now write framing information. It will be needed when
serializing many mutations. The old version was left as
write_without_framing().
This change also makes the serializer interface closer to that of
db::serializer<>. To write an object, the serializer instance is
instantiated with the object and remembers the size, so that size
calculation doesn't have to be repeated again for write().
The immediate motivation for introducing frozen_mutation is inability
to deserialize current "mutation" object, which needs schema reference
at the time it's constructed. It needs schema to initialize its
internal maps with proper key comparators, which depend on schema.
frozen_mutation is an immutable, compact form of a mutation. It
doesn't use complex in-memory strucutres, data is stored in a linear
buffer. In case of frozen_mutation schema needs to be supplied only at
the time mutation partition is visited. Therefore it can be trivially
deserialized without schema.
