This is a follow-up on the `go fix` run in commit
0210c5fd92.
That commit recorded the automated updates.
This one applies further simplifications that are mechanical and do not
involve logic changes, but are outside of what `go fix` did.
It also removes some now-obsolete comments and statements.
This commit introduces a new tracing library, that replaces
golang.org/x/net/trace, and supports (amongts other thing) nested
traces.
This is a minimal change, future patches will make use of the new
functionality.