Fixes#10489
Killing the CDC log table on CDC disable is unhelpful in many ways,
partly because it can cause random exceptions on nodes trying to
do a CDC-enabled write at the same time as log table is dropped,
but also because it makes it impossible to collect data generated
before CDC was turned off, but which is not yet consumed.
Since data should be TTL:ed anyway, retaining the table should not
really add any overhead beyond the compaction to eventually clear
it. And user did set TTL=0 (disabled), then he is already responsible
for clearing out the data.
This also has the nice feature of meshing with the alternator streams
semantics.
Closes#10601
adda43edc7