Support monitoring of the blaze-client pool

[rossabaker]

• Exposes some basic metrics on the client pool for monitoring. Our use case is to see that the wait queue is backing up before it pops.
• Introduces a ConnectionManager.Stateful. The only production implementation, PoolManager, is this.
• Similar to EmberClient, though I didn't name the tuple.
• Less similar to AsyncHttpClientStats, because we don't retain an underlying backend object to obtain stats separately

[zmccoy]

Thanks for doing this! 🎊