rewrite the function as coroutine to make it easier to read and maintain, following lifetime issues we had and fixed in this function.
The second commit adds a test that drops a table while there is a counter update operation ongoing in the table.
The test reproduces issue https://github.com/scylladb/scylla-enterprise/issues/4475 and verifies it is fixed.
Follow-up to https://github.com/scylladb/scylladb/pull/19948
Doesn't require backport because the fix to the issue was already done and backported. This is just cleanup and a test.
Closesscylladb/scylladb#19982
* github.com:scylladb/scylladb:
db: test counter update while table is dropped
db: coroutinize do_apply_counter_update
a038a1fdef