internal/transport: minor cleanup of controlBuffer code

[easwars]

This PR makes a few minor cleanups to the controlBuffer code with the aim of making it more readable.

Summary of changes:

• Rename ch to wakeupCh
  • This channel is used to wake up readers blocked waiting for something to read out of the control buffer, and ch simply isn't descriptive enough, since the scope of this field is reasonable large.
• Got rid of the err field with a closed boolean
  • The only time when the control buffer returns error is when it is closed and the error value returned in this case is always ErrConnClosing. Using a closed boolean and explicitly returning ErrConnClosing makes this explicit, instead of making the reader wonder what possible error values could be returned.
• Changed the type of trfChan from atomic.Value to atomic.Pointer
  • The latter is ergonomic to use and we have started using it in a lot of places in our codebase (in newer code).
• Got rid of the execute method which had a single caller, and change the caller to work with executeAndPut
• Switched all methods to defer the call to cc.Unlock
  • This eliminates multiple calls to cc.Unlock from each of the return paths
  • Over the years, the Go team has made significant improvements in defer performance. So, it is no longer required that we avoid defer in the hopes for improved performance, but in return having harder to read code.
• Added comments to all methods of controlBuffer and indented existing comment lines at 80 cols.

RELEASE NOTES: none

[arvindbr8]

Great! thanks so much for looking into this. Hopefully your eyes didnt need bleaching..

Just a few comments. Please let me know when its ready for another review.

[arvindbr8]

Thanks!