Per client retry-budget

[damb]

Drop requests to data centers (DC) not available.

Implement a endpoint request filtering algorithm based on a per client retry-budget. The algorithm is described within Site Reliability Engineering (Beyer et al.). The retry budget is based on a distributed HTTP response code time series (Redis). An error ratio is computed by means of an rolling window approach where status codes are constantly updated and periodically removed based on a configurable TTL.

Currently only endpoint HTTP status codes 500 and 503 are considered as errors (in future error status codes might be configurable, too).

Features and Changes:

• HTTP response code time series with a URL netloc:path granularity (e.g. netloc=eida.ethz.ch:80, path=/fdsnws/station/1/query)
• Configurable rolling window size
• Configurable TTL for status codes within the response code time series; the TTL defines when requests should be forwarded to endpoints, again
• Configurable per client retry-budget

Note: This PR implements a per client retry-budget, only. At the time being a per (endpoint-)request retry-budget is not used.

[damb]

Unittests with respect to the response code stats are still missing.