* Refactor HTTP code into a common utility class
Centralizes the six (slightly) different implementations to use
a common Apache HTTP Client 5 implementation which implements our
strategies for retries, timeouts, error handling, etc.
Apache HTTP Client 5 adds support for Retry-After headers, HTTP/2,
and a bunch of other stuff under the covers.
Moves request delay to a request interceptor and fixes calculation
of the delay (again). Increase retries from 1x to 3x and use delay*2
as the default retry interval, if no Retry-After header. Uses an
exponential backoff strategy for multiple retries.
* Reuses HTTP client across requests
* Use IOException instead of Exception for HTTP errors
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