Make the healthcheck http verb configurable

[SEJeff]

HEAD requests use less resources, so they can be executed more frequently.

An example configuration would look like:

                            "share_adjusters": [
                                {   
                                    "healthy_threshold": 2,
                                    "interval": 3,
                                    "port_name": "http",
                                    "route": "/health",
                                    "share_adjuster_class": "tellapart.aurproxy.share.adjusters.HttpHealthCheckShareAdjuster",
                                    "timeout": 5,
                                    "unhealthy_threshold": 2,
                                    "http_method": "HEAD"
                                }   
                            ]

I'd like your thoughts on the best way to write some tests for this before merging it however. There are some very simple tests for the adjusters, but nothing for the healthcheck adjusters. I might have more time to try to dig into writing some tests for this in the evenings, but this certainly works from my testing.

[SEJeff]

FWIW, I setup aurproxy to healthcheck using this patch every 3 seconds, and have put ~600k requests through it with no problems at all. Works like a champ.

[ThanosBaskous]

lgtm. Thanks!