Return stats values as numbers

If the returned value of a stat is a number, return it as a
number and not as a string.

Change-Id: Ic5826ef651e40ffe525b70cc3f856a143704b6f7
Reviewed-on: http://review.couchbase.org/25877
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Cosmin Luță <q4break@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>