On the fsfs-format7:  Significantly speed up l2p and p2l index parsing.

First of all, reading numbers one byte at a time is extremely expensive.
We will therefore introduce a buffering stream-y object that reads and
decodes a whole bunch of numbers at a time while still providing full
random access to the underlying data.

Secondly, we try to reuse open files / streams between different stages
of index access, i.e. header and page access.

* subversion/libsvn_fs_fs/index.c
  (MAX_NUMBER_PREFETCH,
   value_position_pair_t,
   packed_number_stream_t): new types and definitions
  (packed_stream_read,
   packed_stream_open,
   packed_stream_close,
   packed_stream_get,
   packed_stream_seek,
   packed_stream_offset): implement the packed numbers stream
  (read_number): drop; superseded by packed numbers stream
  (get_l2p_header,
   get_l2p_page,
   l2p_index_lookup,
   svn_fs_fs__l2p_get_max_ids,
   get_p2l_header,
   read_entry,
   get_p2l_page,
   svn_fs_fs__p2l_index_lookup): switch to stream; reuse open streams

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1438988 13f79535-47bb-0310-9956-ffa450edef68