More signed/unsigned conversation warning fixes.

* subversion/libsvn_ra_serf/update.c
  (get_best_connection): Use proper types for connection number and 
   requests count.

* subversion/tests/libsvn_fs_fs/fs-fs-private-test.c
  (verify_representation_stats, verify_node_stats): Change EXPECTED_COUNT 
   argument type to apr_uint64_t.
  (verify_histogram): Change SUM_COUNT and SUM_SIZE local variable type to 
   apr_uint64_t.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1647549 13f79535-47bb-0310-9956-ffa450edef68