pop3: speed up STAT slightly (~1%)

pop3: speed up STAT slightly (~1%)

We can calculate the total size of the mailbox while generating
the cache, which allows us to iterate the cache again to
calculate the size of the mailbox slice.  While we're in the
area, simplify the loop and avoid needlessly updating the `$beg'
variable.

This adds a small amount of constant time overhead to DELE,
however that is amortized across multiple requests for fairness.