rebar: Add nowarn_deprecated_type to let Disco compile with Erlang 17.
ddfs master: Refactor ddfs_master to respect the opaqueness of gb_sets().

Basically, instead of using the false|gb_set() for tag_cache we now use
{boolean(), gb_set()} where the first item shows whether the cache is valid or
not.
See: http://erlang.org/pipermail/erlang-questions/2014-April/078647.html
doc: Add a note about Erlang 17.
Merge pull request #490 from pooya/r17

R17