This commit adds support to pass a preimage selected by an upper layer
to CDC. The responsibility for the correctness of the preimage (i.e. the
selected columns, whether it's up to date, etc.) lies with the caller.
It may be improved in the future by validating the preimage, e.g. by
"slicing" the received preimage to the necessary columns.
The motivation behind this change was to reduce the number of
read-before-writes and avoid reading the row twice for Alternator
Streams in an increased compatibility mode with DynamoDB. This is to be
added in a following commit. Until now, this commit should be a no-op.
d4581cc442