Parallelize reference resolution

[sethvargo]

While the parser de-duplicates any references, the current resolution occurs in serial. This is fine for a small number of references, but it can take unnecessarily long for many. We should parallelize the reference lookup using the semaphore pattern.

Design

1. Add new flag -concurrency to update and pin commands. Default to max(numCPUs()-1, 1), plumb through.

2. Create a semaphore of concurrency and background the go routines:

   for _, ref := range references {
     if err :- sem.Acquire(1); err != nil {
       return err
     }
   
     go func() {
       if err := ref.Resolve(); err != nil {
         // handle error
       }
     }()
   }
   
   if err := sem.Acquire(max); err != nil {
     return err
   }
   
   // handle goroutine errors