Add stat for "number of items in last commit"

While diagnosing disk performance issues, it's sometimes very useful to know specifically how many items were included in the last "commit" to sqlite.

I imagine we will have a similar concept with CouchDB, so this effort will not go wasted.

For example, when our max_txn_size is set to 10,000...the software may actually commit anywhere from 1 to 10,000 items depending on the size of the queue. We want to know this number...