Automatic merge from submit-queue

pkg/util/goroutinemap: apply idiomatic Go cleanups

Package goroutinemap can be structurally simplified to be more
idiomatic, concise, and free of error potential.  No structural changes
are made.

It is unconventional declare `sync.Mutex` directly as a pointerized
field in a parent structure.  The `sync.Mutex` operates on pointer
receivers of itself; and by relying on that, the types that contain
those fields can be safely constructed using
https://golang.org/ref/spec#The_zero_value semantic.

The duration constants are already of type `time.Duration`, so
re-declaring that is redundant.

/CC: @saad-ali