support MaxConnsPerHost

[jranson]

• in developer env use host directory for reds and prom docker compose volumes.
• updates health checks to use reasonable connection timeouts and limit to 1 health check connection per http hostname at a time
• supports http.Client's MaxConnsPerHost in backend http clients, exposed as MaxConcurrentConns in the Backend Options, defaults to 20 (same as current MaxIdleConns value). This allows a Trickster operator to limit the number of concurrent requests to any/all origins (blocking the rest in order of execution) in order to minimize the impact of cache miss storms.

[coveralls]

Pull Request Test Coverage Report for Build 20320892879

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 72.144%

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Totals
Change from base Build 20318433563:	0.002%
Covered Lines:	14933
Relevant Lines:	20699

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💛 - Coveralls