The partition based splitting writer (used by scrub) was found to be exception-unsafe, converting an `std::bad_alloc` to an assert failure. This series fixes the problem and adds a unit test checking the exception safety against `std::bad_alloc`:s fixing any other related problems found.
Fixes: https://github.com/scylladb/scylla/issues/9452Closes#9453
* github.com:scylladb/scylla:
test: mutation_writer_test: add exception safety test for segregate_by_partition()
mutation_writer: segregate_by_partition(): make exception safe
mutation_reader: queue_reader_handle: make abandoned() exception safe
mutation_writer: feed_writers(): make it a coroutine
mutation_writer: partition_based_splitting_writer: erase old bucket if we fail to create replacement