Add Retry and Ratelimit decorators

[prkumar]

#102 #71

Changes proposed in this pull request:

• Adds a ratelimit decorator to constrain consumers to making some maximum number of calls within a given time period.
• Adds a retry decorator to enable reattempts of failed requests. The API for this decorator is heavily influenced by jd/tenacity.
• Backend: Adds a request execution state machine that facilitates building logic like retrying, rate-limiting, and circuit breaking in an io-agnostic way. This means that the retry and ratelimit decorator should work for both blocking and non-blocking (e.g., Asyncio, Twisted) I/O!

Attention: @prkumar

[codecov]

Codecov Report

Merging #132 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #132    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files          25     35    +10     
  Lines        1425   1853   +428     
  Branches      136    155    +19     
======================================
+ Hits         1425   1853   +428

Impacted Files	Coverage Δ
uplink/retry.py	100% <100%> (ø)
uplink/clients/io/asyncio_strategy.py	100% <100%> (ø)
uplink/clients/io/__init__.py	100% <100%> (ø)
uplink/clients/io/transitions.py	100% <100%> (ø)
uplink/clients/io/templates.py	100% <100%> (ø)
uplink/clients/io/state.py	100% <100%> (ø)
uplink/clients/requests_.py	100% <100%> (ø)	⬆️
uplink/clients/io/blocking_strategy.py	100% <100%> (ø)
uplink/builder.py	100% <100%> (ø)	⬆️
uplink/clients/io/twisted_strategy.py	100% <100%> (ø)
... and 14 more

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