bfopen: save individual series' original metadata

Previously the union of all series' original metadata was stored once
for each series.  This causes potentially substantially more memory to
be used, and makes it more difficult to read original metadata.
bfopen: minor code cleanup
Merge pull request #587 from melissalinkert/rebased/dev_4_4/bfopen-metadata

bfopen: save individual series' original metadata (rebased onto dev_4_4)