On the fsfs-format7 branch: optimize P2L index read code.  Open the input
stream only once and fine-tune prefetching.

* subversion/libsvn_fs_fs/index.c
  (get_p2l_page_info): add STREAM_POOL parameter; delay result allocation
  (get_p2l_page): add STREAM_POOL parameter and pass it through
  (prefetch_p2l_page): ditto; use "leaking bucket" pattern to determine
                       whether we should continue to prefetch data
  (svn_fs_fs__p2l_index_lookup): update caller; read more linearly


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1476656 13f79535-47bb-0310-9956-ffa450edef68