Addition of cdc column in scylla_tables changes how schema
digests are calculated, and affect the ABI of schema update
messages (adding a column changes other columns' indexes
in frozen_mutation).
To fix this, extend the schema_tables mechanism with support
for the cdc column, and adjust schemas and mutations to remove
that column when sending schemas during upgrade.
Signed-off-by: Piotr Jastrzebski <piotr@scylladb.com>