Fix race in client cancellation propagation

[marcoferrer]

Address race condition when prematurely cancelling a call via closing its outbound channel.

Issue reported in #93

[codecov]

Codecov Report

Merging #98 into master will increase coverage by 0.67%.
The diff coverage is 64.7%.

@@             Coverage Diff             @@
##             master     #98      +/-   ##
===========================================
+ Coverage     86.53%   87.2%   +0.67%     
  Complexity       19      19              
===========================================
  Files            15      15              
  Lines           312     344      +32     
  Branches         53      47       -6     
===========================================
+ Hits            270     300      +30     
- Misses           15      26      +11     
+ Partials         27      18       -9

Impacted Files	Coverage Δ	Complexity Δ
...ithub/marcoferrer/krotoplus/coroutines/RpcUtils.kt	92.3% <ø> (+29.8%)	0 <0> (ø)	⬇️
...us/coroutines/client/ClientStreamingCallChannel.kt	81.81% <100%> (+4.04%)	0 <0> (ø)	⬇️
...oferrer/krotoplus/coroutines/server/ServerCalls.kt	87.2% <100%> (+5.5%)	0 <0> (ø)	⬇️
...rcoferrer/krotoplus/coroutines/call/FlowControl.kt	72.91% <45%> (-16.28%)	0 <0> (ø)
...otoplus/coroutines/client/ClientBidiCallChannel.kt	96.66% <87.5%> (+1.01%)	0 <0> (ø)	⬇️
...rrer/krotoplus/coroutines/server/ServerChannels.kt	90% <0%> (+1.11%)	0% <0%> (ø)	⬇️
... and 2 more

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