Fix review comments: R16B02 use, multi-part support

Also: Fix bug in FS2 rate_a_dict: ?MD_DELETED may be "true"

Also: Fix GC block delete errors

Also: Fix overlooked BClass bug in block_sequences_for_part_manifests_keep()

Also: Refactor fixup_a_part_manifest() and friends, fix part reordering bug!

Also: add an Erlang test to try to trigger multipart reassembly order bug.
Add pb_backlog section to README.fs2-backend.txt