Use PRIu64 instead of nonstandard %llu

couch-kvstore.cc:2376:43: warning: format '%llu' expects argument of type
                          'long long unsigned int', but argument 6 has
                          type 'uint64_t {aka long unsigned int}' [-Wformat=]

Change-Id: I7b67abf8d17559d8d8b6f9eb0dece58238514cef
Reviewed-on: http://review.couchbase.org/42232
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>