make the initial stream / connection flow control windows configurable

[marten-seemann]

This is primarily to make 0-RTT better testable when we relax the resumption condition, see #3061.

As we're shifting more responsibility for setting these values to the application, we could say this fixes #88.

Should we rename the config options? The current ones are a bit of a mouthful. Suggestion:
Initial{Stream, Connection}FlowControlWindow => Initial{Stream, Connection}ReceiveWindow
MaxReceive{Stream, Connection}FlowControlWindow => Max{Stream, Connection}ReceiveWindow

[codecov]

Codecov Report

Merging #3083 (a15cfd6) into master (9dcb56b) will increase coverage by 0.02%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master    #3083      +/-   ##
==========================================
+ Coverage   86.03%   86.05%   +0.02%     
==========================================
  Files         132      132              
  Lines        9441     9449       +8     
==========================================
+ Hits         8122     8131       +9     
+ Misses        957      956       -1     
  Partials      362      362              

Impacted Files	Coverage Δ
session.go	76.60% <90.00%> (ø)
config.go	100.00% <100.00%> (ø)
internal/utils/rand.go	75.00% <0.00%> (+12.50%)	⬆️

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[lucas-clemente]

Renaming sgtm.