Replace `this` idiom with `mut self`.
Move idle connections constants from pool.rs to agent.rs.
Remove Agent.set and some convenience request methods
(leaving get, post, and put).
Move max_idle_connections setting from AgentConfig to AgentBuilder
(since the builder passes these to ConnectionPool and the Agent
doesn't subsequently need them).
Eliminate duplicate copy of proxy in AgentState; use the one in
AgentConfig.
aae05c5614