Rust connpool: Implement an explicit reconnection phase

[mmastrac]

We want to track reconnections for QoS and algorithm purposes -- we want to account for the time spend disconnecting and reconnecting when calculating the expense of switching connections.

[fantix]

Thanks @mmastrac for the explanation in PM! So basically:

• Each connect and each reconnect has a matching disconnect event

• The calculataion for how long it takes to reconnect is just the greatest of

  1. the actual average reconnection time (which is a disconnect + connect) and
  2. the average times to connect and disconnect.

  This just ensures we can get that number even when our stats are sparse. Eventually, sum(avg(connect) + avg(disconnect) should roughly equal avg(reconnect) once there's enough data.