Add num_done counter to the pg_stat_checkpointer view.

Add num_done counter to the pg_stat_checkpointer view.

Checkpoints can be skipped when the server is idle. The existing num_timed and
num_requested counters in pg_stat_checkpointer track both completed and
skipped checkpoints, but there was no way to count only the completed ones.

This commit introduces the num_done counter, which tracks only completed
checkpoints, making it easier to see how many were actually performed.

Bump catalog version.

Author: Anton A. Melnikov
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/9ea77f40-818d-4841-9dee-158ac8f6e690@oss.nttdata.com