device: split IpcSetOperation into parts

device: split IpcSetOperation into parts

The goal of this change is to make the structure
of IpcSetOperation easier to follow.

IpcSetOperation contains a small state machine:
It starts by configuring the device,
then shifts to configuring one peer at a time.

Having the code all in one giant method obscured that structure.
Split out the parts into helper functions and encapsulate the peer state.

This makes the overall structure more apparent.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>