Expose Stream.die via Stream.GetDieCh for canceling running golang RPC func

[auxten]

We are working on a Blockchain featured SQL database: CovenantSQL
Because golang net/rpc did not provide a context. We have no way to get notified when something happened to the underlying TCP connection to cancel a running RPC.
Thanks to xtaci/smux, it is possible with Stream.die exposed.

[codecov-io]

Codecov Report

Merging #40 into master will increase coverage by 0.85%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   85.56%   86.42%   +0.85%     
==========================================
  Files           4        4              
  Lines         381      383       +2     
==========================================
+ Hits          326      331       +5     
+ Misses         49       47       -2     
+ Partials        6        5       -1

Impacted Files	Coverage Δ
stream.go	82.87% <100%> (+0.23%)	⬆️
session.go	87.62% <0%> (+1.54%)	⬆️

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[xtaci]

I recommend to add a function to return s.die instead of exposing the variable directly

[xtaci]

and also make it return a recv only chan

[auxten]

I recommend to add a function to return s.die instead of exposing the variable directly and also make it return a recv only chan

Good advise, done.