To support our test suite, teach the repos pool to optionally
require a specific repository UUID and to re-read from disk
upon mismatch.  This allows our test suite to replace repositories
without restarting the server.

* subversion/include/private/svn_repos_private.h
  (svn_repos__repos_pool_get): add UUID parameter

* subversion/libsvn_repos/repos_pool.c
  (svn_repos__repos_pool_get): update implementation

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1541293 13f79535-47bb-0310-9956-ffa450edef68