Move arena_copy_buf method to util.cc

Change-Id: I2a80750038ce110a41f6903295f0655ae27468dc
Reviewed-on: http://review.couchbase.org/31660
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>