de-/serialization fixes/improvements

Hi Ryan,

I have encountered a few bugs in fastserializer.go as explained in the pull request I recently closed
To summarize:
* de-serialization of null strings resulted in panic. Fixed by de-serializing as empty strings (string's zero-value).
* TIMESTAMP de-/serialization had a rather ugly bug that broke de-serialization of TIMESTAMP before 1970-01-01.
* Changed TIMESTAMP to use voltdb's NULL for time.Time{}.

What do you think?