On the fsfs-format7 branch:  Represent the "next IDs" counters internally
as the unsigned integers that they are.  At that point, we can drop the
key-gen functionality that operated on base36 strings.

This is the first larger step in the "numerical IDs" patch series.

* subversion/libsvn_fs_fs/util.h
  (write_current): expect integers for node and copy IDs
  (get_file_offset): fix a comment string

* subversion/libsvn_fs_fs/util.c
  (write_current): update implementation to convert unsigned to base36

* subversion/libsvn_fs_fs/transaction.c
  (get_and_increment_txn_key_baton,
   get_and_increment_txn_key_body,
   create_txn_dir): do txn ID arithmetics on ints instead of strings
  (write_next_ids,
   read_next_ids): convert int <-> base36 at the file interface
  (get_new_txn_node_id,
   svn_fs_fs__reserve_copy_id,
   get_next_revision_ids): update callers; simplify
  (get_final_id): new utility function
  (write_final_rev): update caller; global IDs are now ints
  (write_final_current,
   commit_body): update callers

* subversion/libsvn_fs_fs/hotcopy.c
  (hotcopy_update_current): update caller

* subversion/libsvn_fs_fs/recovery.h
  (svn_fs_fs__find_max_ids): for IDs, use integers instead of strings

* subversion/libsvn_fs_fs/recovery.c
  (count_from_id): new utility function
  (recover_find_max_ids,
   svn_fs_fs__find_max_ids,
   recover_body): use integers to record the largest IDs

* subversion/libsvn_fs_fs/key-gen.c
  (): drop file

* subversion/libsvn_fs_fs/key-gen.h
  (): drop file

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1453765 13f79535-47bb-0310-9956-ffa450edef68