Allow configuring dispatcher 'maxRequestsPerHost'

[checketts]

In developing some code that produces/consume a lot of message, I found that the default appender would get backed up, causing log messages to queue up, chewing up memory until the application crashed. This appears to have been due to the default OKHttp dispatcher only allowing 5 calls 'per host' at a time.

I added the ability to access the Dispatcher from the Logback appender in the code and was able to set the maxRequestsPerHost to 30 and that fixed my problem.

Would that change be welcome as a PR?

Exposing the Dispatcher also gave me access to the ready and running queue sizes, so I was able to set those up as metrics and get much better insight into what was clogging the system and how I might address it.