cleanup: rename fields for clarity

[dfawley]

Mostly around the "stream" types. Also some cryptically named fields in ClientConn/etc and local variables.

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 84.37500% with 20 lines in your changes missing coverage. Please review.

Project coverage is 82.34%. Comparing base (9d4fa67) to head (ceaf641).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
stream.go	82.14%	12 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8043      +/-   ##
==========================================
- Coverage   82.35%   82.34%   -0.01%     
==========================================
  Files         383      383              
  Lines       38776    38776              
==========================================
- Hits        31933    31932       -1     
+ Misses       5532     5531       -1     
- Partials     1311     1313       +2     

Files with missing lines	Coverage Δ
clientconn.go	92.45% <100.00%> (-0.27%)	⬇️
dialoptions.go	90.80% <100.00%> (ø)
rpc_util.go	80.67% <100.00%> (ø)
server.go	82.62% <100.00%> (+0.62%)	⬆️
stream.go	81.69% <82.14%> (ø)

... and 18 files with indirect coverage changes