Metadata from upload endpoints should only set null fields

Fix issue updating persisted config

Engine Placer

Lookup jobs at session level

Have pip update itself in virtualenv first

Don't modify the dict when loading a job

Add prefetch feature, defaults to off

Add bulk file download

fix bug on join search

use _source when filter_on_field is not _id
add some logging

On macOS El Capitan, the virtualenv created uses an older version of pip which must compile the cryptography wheel. This fails per pyca/cryptography#2350. Having the virtualenv pip update itself before running resolves this issue which prevents ./bin/run.sh from completing "Installing Python requirements" on macOS 10.11.