MB-7656 Make file arg for read APIs const

It doesn't need to be non-const. This makes the API more clear.

Change-Id: I6944e294abf625f5da42c50f09d8827ac1cc2159
Reviewed-on: http://review.couchbase.org/28791
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>