test: raft log rollback

Also, fix the code to pass the test:

1. Use replace instead of insert when appending entries. They may be overridden.
2. Increase pool worker queue size. Raft node may send several messages within a single tick. Without this change the second message is dropped.

The second change is a poorly designed workaround. Constant limit doesn't seem to be both cost-effective and reliable at the same time, but we don't have a better solution at the moment.

Close: https://gitlab.com/picodata/picodata/picodata/-/issues/26