Support retrying gRPCs with chained interceptors

[marcwilson-g]

As retries may cause a handler chain to be partially re-executed the
interceptor needs to be reset as the recursion unrolls.

Fix for #110

Possibly regressed as part of
5d4723c

Added tests to the retry_test as this case seemed retry specific, but did feel odd to test the chaining in the retry.

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[codecov-io]

Codecov Report

Merging #100 into master will increase coverage by 0.47%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage    73.2%   73.68%   +0.47%     
==========================================
  Files          36       36              
  Lines        1284     1292       +8     
==========================================
+ Hits          940      952      +12     
+ Misses        295      293       -2     
+ Partials       49       47       -2

Impacted Files	Coverage Δ
chain.go	77.27% <100%> (+2.27%)	⬆️
retry/retry.go	74.09% <0%> (+1.2%)	⬆️
retry/backoff.go	100% <0%> (+33.33%)	⬆️

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[marcwilson-g]

I signed it!

[googlebot]

CLAs look good, thanks!

[changpingc]

ping @alcore can you review this? thanks

[domgreen]

@marcwilson-g could you update the documentation explicit that the retry will call every interceptor after the retry when a retry happens.
Can you do this in both retry/doc.go and doc.go.

[domgreen]

@yifanzz @mwitkow LGTM apart from updating docs 👍

[marcwilson-g]

Back from leave today!
Have added comments in both retry/doc.go and doc.go as suggested (though it wasn't clear if I should also add comments to the .md files too as they seem to contain duplicate content to the doc.go files?

[domgreen]

Hey Marc, sorry for the delay getting back to you. The if you run make it should auto generate the .MD files for you.

[marcwilson-g]

@domgreen Done!

[domgreen]

Thanks Marc, will chase someone to accept the PR :)

[hxzhao527]

Why sever side also need curI--?