Adds set_max_idle_connections and set_max_idle_connections_per_host.
This turns the values of Pool.recycle into a VecDeque of Streams for the same PoolKey.
The freshest stream (most recently used) is at the back; the stalest stream is at the front.
This also removes the invariant "Each PoolKey exists in recycle at most once and lru at
most once," replacing it with "each PoolKey has the same number of entries in lru as in
recycle."
Fixes#110
Alex L