IO::FlatDatastore: protected release_stream() from exceptions by std::fstream::close().
IO::FlatDatastore: bound-append obj_id_str instead of relying on NUL.
dep: bumped zeromq to 4.0.1.
utility: import from duct++ instead of Hord.
IO::FlatDatastore: updated PropInfo ctor, misc pedantry.
Updated to Hord error reporting changes, added Onsang's own.