Adapt couchstore_savedocs API so that it can tell whether each item persisted is new or existing

With the full eviction support, there are some difficulties in maintaining the total number of valid items per vbucket including dirty items + new items that are not persisted in couchstore. For the accurate counting, we need the following information every time the ep-engine does the batch commit:

For each item within the batch commit,

For SET, Is the item a new insertion or update in couchstore?
For DEL, Is the item a new deletion (i.e., non-existent item deletion) or existent item deletion in couchstore?

For this, we need to pass the following callback function to couchstore_savedocs API

typedef int (*couchstore_savedocs_callback_fn)(Db *db,
DocInfo *docinfo,
bool isNew,
void *ctx);

If docinfo.deleted == true && isNew == true, it means the non-existent item deletion.