Queues: unified, nicer runner method

[anshumanmohan]

When hacking on SDN stuff, I came up with a nicer top-level runner method that I wanted all the other queues (FIFO, PIFO, PIFO tree) to start using as well. The important change is that:

Before

The main component would:

• Have access to external memories and be passed a queue component.
• Pass all the commands to the queue component in sequence.
• Stash any answers (gotten from popping or peeking) into the answer memory.

After

The main component:

• Has access to external memories and is passed:
  • A queue component.
  • An optional controller component.
  • An optional stats component.
• It sets up runner component that thereafter serves as the handle to the queue component. If main was passed a stats component, it sets up the runner to also use that stats component. The runner has the list of commands, as is in charge of feeding the queue one command at each invocation of the runner. The runner also returns to its caller any answers (from popping or peeking) and raises errors as needed.
• In a loop, it invokes the runner until the runner raises an error. If passed a controller component, it invokes the controller component every time it invokes the runner. Regardless, the loop runs until the failure of runner.

The SDN clients use the main component with all the optional fields filled, while the lighter-weight FIFO, PIFO, PIFOtree just use it without those fields.

Closes #1770.