extend and improve the tests of virtual tables for cdc with tablets.
split the existing virtual tables test to one test that validates the
virtual tables against the internal cdc tables, and triggering some
tablet splits in order to create entries in the cdc_streams_history
table, and add another test with basic validation of the virtual tables
when there are multiple cdc tables.
add a background fiber to the topology coordinator that runs
periodically and checks for old CDC streams for tablets keyspaces that
can be garbage collected.
Introduce basic tests creating CDC tables in tablets-enabled keyspaces,
verifying we can create and drop CDC tables, write and consume CDC log
entries, and consume the log while splitting streams.
