uv_append: Don't finalize segment if there are writes in-flight

[MathieuBordere]

calling uvAliveSegmentFinalize(s) while there are writes against the alive segment in flight could lead to uvWriterClose being called twice, leading to a triggered assertion src/uv_writer.c:393: UvWriterClose: Assertion '!w->closing' failed.

This can happen when e.g.:

• write against open segment completes in uvAliveSegmentWriteCb
• write triggers a checkpoint
• checkpoint triggers a raft apply command
• triggers a write against the open segment
• adds write to poll_queue
• raft apply command triggers a snapshot
• snapshot triggers finalization of open segment
• causes uvAliveSegmentFinalize(s) to be called here
• leading to the cancellation of requests in the poll_queue
• leading to a failed write and UvWriterClose being called again.

To avoid this, no longer finalize the alive segment if there is a write in-flight, checked by comparing pending_last_index & last_index on the segment. When the in-flight write completes, the segment will be finalized.