(WIP) Add @circuit_breaker decorator

[prkumar]

Changes proposed in this pull request:

• @circuit_breaker decorator: decorating a Consumer subclass with this annotation adds a Circuit Breaker that monitors request(s) and improves the resilience of the client against cascading failures caused by downtime of the consumed service.

[codecov[bot]]

Codecov Report

Merging #156 into master will decrease coverage by 13.06%. The diff coverage is 0%.

@@             Coverage Diff             @@
##           master     #156       +/-   ##
===========================================
- Coverage     100%   86.93%   -13.07%     
===========================================
  Files          41       42        +1     
  Lines        2122     2441      +319     
  Branches      167      177       +10     
===========================================
  Hits         2122     2122               
- Misses          0      319      +319

Impacted Files	Coverage Δ
uplink/circuit_breaker.py	0% <0%> (ø)

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[codecov[bot]]

Codecov Report

Merging #156 into master will decrease coverage by 12.92%. The diff coverage is 0%.

@@             Coverage Diff             @@
##           master     #156       +/-   ##
===========================================
- Coverage     100%   87.07%   -12.93%     
===========================================
  Files          41       42        +1     
  Lines        2175     2468      +293     
  Branches      173      182        +9     
===========================================
- Hits         2175     2149       -26     
- Misses          0      319      +319

Impacted Files	Coverage Δ
uplink/circuit_breaker.py	0% <0%> (ø)
uplink/arguments.py	100% <0%> (ø)	:arrow_up:
uplink/retry/retry.py	100% <0%> (ø)	:arrow_up:
uplink/ratelimit.py	100% <0%> (ø)	:arrow_up:
uplink/builder.py	100% <0%> (ø)	:arrow_up:
uplink/retry/stop.py	100% <0%> (ø)	:arrow_up:
uplink/commands.py	100% <0%> (ø)	:arrow_up:
uplink/helpers.py	100% <0%> (ø)	:arrow_up:
uplink/converters/marshmallow_.py	100% <0%> (ø)	:arrow_up:

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[cognifloyd]

I can think of several instances, recently, where a circuit breaker would have been very helpful due to a misbehaving service that was giving itself the DDoS treatment. :p Very Cool!