On the log-addressing branch:  Add minimal packing support for logically
addressed shards to PASS more tests.

This is not transient code but will always be needed as a fallback for
extremly large revisions.  It also introduces the "pack context"
infrastructure.  Later revisions will add reordering logic.

* subversion/libsvn_fs_fs/pack.c
  (pack_context_t
   initialize_pack_context,
   reset_pack_context,
   close_pack_context): introduce packing context context infrastructure
  (copy_file_data): utility to efficiently (space and time) copy a data
                    block from one file to another
  (append_revision,
   pack_log_addressed): non-reordering packing logic log. addressing
  (pack_phys_addressed): tweak docstring
  (pack_rev_shard): call the new logic for logically addressed shards

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1512244 13f79535-47bb-0310-9956-ffa450edef68