Document basic backoff protocol.
Add example "429 Too Many Requests" response to the docs.
Remove Backoff header from protocol docs.
Warn but don't die if using MemoryNonceDB in production.
Merge pull request #323 from mozilla/rfk/basic-backoff

Document basic backoff protocol.
Merge pull request #328 from mozilla/rfk/remove-noncedb-kill-switch

Warn but don't die if using MemoryNonceDB in production.