[Sync] Move some directory-related things from core/ to syncable/.

Everything is being moved out of core to better homes. Things moved:

syncable/
  base_node
  base_transaction
  change_record
  change_reorder_buffer
  delete_journal
  read_node
  read_transaction
  test_user_share
  user_share
  write_node
  write_transaction

My knowledge of the role each of these plays is not perfect, so let me
know if something should not have been moved.

BUG=631271

Review-Url: https://codereview.chromium.org/2407163004
Cr-Commit-Position: refs/heads/master@{#424743}
60 files changed