Arista: port NX-OS route-map logic

Prior to this PR, Arista modeled route-maps as nested structures, so that if
there were N terms, those terms would go into a recursive structure that was N
terms deep. For large N (hundreds+) this can cause recursion limits in Java,
e.g., in walking the route-map or in serializing it to disk.

Fix this by adopting (neearly verbatim) the logic from NX-OS.

Validated by existing unit tests + Batfish Enterprise compare view: routes are
the same before and after in many real networks.