Improve speed of source uploads

[bfhealy]

Currently scope_upload_classification.py can upload ~1000 sources/classifications/light curves/annotations to Fritz per hour if two instances are run simultaneously on independent parts of source lists (using the -start -stop arguments). The server starts to complain if three or more instances are run together. See also #89.

Feature Summary
With the potential for millions of sources to be uploaded going forward, we should determine how much our code can be streamlined to make uploads complete faster.

Implementation details
Anything within our loop over each source is a good target for speed enhancements.

[bfhealy]

With the golden dataset uploaded to Fritz (see #57), I am closing this issue for now. It may be reopened once we are confronted with even larger sets of sources to upload to Fritz. Whitelisting the uploader's IP address may be a future enhancement of upload speed.