code: support rewriting labels

[heyitsanthony]

Lets a failpoint to jump to a label only used by failpoints.

/cc @gyuho

[gyuho]

@heyitsanthony

Is there any way that we can release the lock (from defer Acquire) on continue? Otherwise, for {} will keep running continue without release, so cannot run DELETE request. My attempt was add https://github.com/coreos/gofail/pull/12/files#diff-81f179a840cfbb903d2dbc4d8c2152a5R66.

Thanks!

[heyitsanthony]

@gyuho OK, defer is broken in general. e.g., this will starve the delete too:

for ctx.Err() == nil {
  // gofail: var abc struct{}
  // fmt.Println("abc")
  whatever()
}

[gyuho]

@heyitsanthony You are right. defer seems broken then. I will create a separate issue.

Thanks!