Introduce a small new private API that allows for recycling root pools
as piloted by svnserve in r1523465.  Provide some basic testing as well.

* subversion/include/private/svn_subr_private.h
  (svn_root_pools__t): declare new API data types
  (svn_root_pools__create,
   svn_root_pools__acquire_pool,
   svn_root_pools__release_pool): declare new API functions

* subversion/libsvn_subr/root_pools.c
  (): new file
  (svn_root_pools__t): define new API data type
  (acquire_pool_internal): new utility function
  (svn_root_pools__create,
   svn_root_pools__acquire_pool,
   svn_root_pools__release_pool): implement new API functions

* subversion/tests/libsvn_subr/root-pools-test.c
  (): tests for the new API

* build.conf
  (root-pools-test): new build target
  (__ALL_TESTS__): add new test as dependency


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1527140 13f79535-47bb-0310-9956-ffa450edef68