repair: Fix race between write_end_of_stream and apply_rows
repair: Fix race between write_end_of_stream and apply_rows

Consider: n1, n2, n1 is the repair master, n2 is the repair follower.

=== Case 1 ===
1) n1 sends missing rows {r1, r2} to n2
2) n2 runs apply_rows_on_follower to apply rows, e.g., {r1, r2}, r1
   is written to sstable, r2 is not written yet, r1 belongs to
   partition 1, r2 belongs to partition 2. It yields after row r1 is
   written.
   data: partition_start, r1
3) n1 sends repair_row_level_stop to n2 because error has happened on n1
4) n2 calls wait_for_writer_done() which in turn calls write_end_of_stream()
   data: partition_start, r1, partition_end
5) Step 2 resumes to apply the rows.
   data: partition_start, r1, partition_end, partition_end, partition_start, r2

=== Case 2 ===
1) n1 sends missing rows {r1, r2} to n2
2) n2 runs apply_rows_on_follower to apply rows, e.g., {r1, r2}, r1
   is written to sstable, r2 is not written yet, r1 belongs to partition
   1, r2 belongs to partition 2. It yields after partition_start for r2
   is written but before _partition_opened is set to true.
   data: partition_start, r1, partition_end, partition_start
3) n1 sends repair_row_level_stop to n2 because error has happened on n1
4) n2 calls wait_for_writer_done() which in turn calls write_end_of_stream().
   Since _partition_opened[node_idx] is false, partition_end is skipped,
   end_of_stream is written.
   data: partition_start, r1, partition_end, partition_start, end_of_stream

This causes unbalanced partition_start and partition_end in the stream
written to sstables.

To fix, serialize the write_end_of_stream and apply_rows with a semaphore.

Fixes: #6394
Fixes: #6296
Fixes: #6414

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla