Add support for reading plugin cached readings

[edaniszewski]

Currently a WIP until all the other dependent work is done.

a new version of synse-grpc will need to be released first, then the tarball here can be cleaned out, along with a few other temporary things.

Will also want to bump the minor version here

#212

[codecov]

Codecov Report

Merging #213 into master will increase coverage by 0.51%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
+ Coverage   82.76%   83.28%   +0.51%     
==========================================
  Files          44       46       +2     
  Lines        1300     1358      +58     
==========================================
+ Hits         1076     1131      +55     
- Misses        224      227       +3

Impacted Files	Coverage Δ
synse/scheme/read.py	100% <ø> (ø)	⬆️
synse/proto/client.py	100% <100%> (ø)	⬆️
synse/__init__.py	100% <100%> (ø)	⬆️
synse/scheme/__init__.py	100% <100%> (ø)	⬆️
synse/commands/__init__.py	100% <100%> (ø)	⬆️
synse/commands/read_cached.py	100% <100%> (ø)
synse/errors.py	100% <100%> (+1.19%)	⬆️
synse/routes/core.py	91.54% <75%> (-2.11%)	⬇️
synse/scheme/read_cached.py	85.71% <85.71%> (ø)
... and 1 more

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