Clarify and unify RPC terminology

Per discussion at !1092 (comment 2894530) and later on a call, we want our RPC code to avoid confusing terminology and misuse. To that end:

• We should say "method", never "command".
• We should refer to "result" only in the spec; in the code it should be a "success" or something.
• "Authentication methods" should become "authentication schemes" or something.