As part of the checkpoint cleanup process, it is possible that the checkpoint can read some of the internal pages of the btree to identify any obsolete child pages. Sometimes this read can slow down the checkpoint operation if there are many pages to be read into the cache. Currently, there are no stats available to find out how many pages are read into the cache by the checkpoint.

As part of this ticket, add statistics to track how many pages are read into the cache by the checkpoint. As the checkpoint reads only the internal pages into the cache, we may not need the bytes read by the checkpoint statistic, as we can find out the approximate bytes by multiplying the pages by the maximum internal page size.