Add basic defer-recover panic handler

[bradleyjkemp]

Ideally the stream interceptor shouldn't panic but if it does (and it doesn't have it's own handler) then do something sensible to prevent grpc-proxy terminating and killing all other RPCs.

[codecov]

Codecov Report

Merging #53 into master will decrease coverage by <.01%.
The diff coverage is 62.5%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   58.21%   58.21%   -0.01%     
==========================================
  Files          30       30              
  Lines        1089     1096       +7     
==========================================
+ Hits          634      638       +4     
- Misses        354      356       +2     
- Partials      101      102       +1

Flag	Coverage Δ
#integration	57.57% <62.5%> (-0.01%)	⬇️
#unit	15.48% <0%> (-0.36%)	⬇️

Impacted Files	Coverage Δ
grpc-proxy/config.go	48.57% <62.5%> (+2.14%)	⬆️

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