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.
The duration constants are already of type `time.Duration`, so
re-declaring that is redundant.
Allow mount volume operations to run in parallel for non-attachable
volume plugins.
Allow unmount volume operations to run in parallel for all volume
plugins.
