refactor: s/peer/server/ in immutable/upload, happinessutil.py, test_upload

refactor: s/peer/server/ in immutable/upload, happinessutil.py, test_upload

No behavioral changes, just updating variable/method names and log messages.
The effects outside these three files should be minimal: some exception
messages changed (to say "server" instead of "peer"), and some internal class
names were changed. A few things still use "peer" to minimize external
changes, like UploadResults.timings["peer_selection"] and
happinessutil.merge_peers, which can be changed later.