Refactor l2_pop code to pass mac/ip info more readably

Previous code used a 2 element array to represent a mac/ip address
pair.  Code assumed that element 0 was mac and 1 was ip.  This made
the code difficult to read and difficult to maintain.  An attempt was
made to insert a third value that failed miserably because of the
position dependence and other code that assumed not only positions but
also the number of elements.

Using a namedtuple seems to be the best way to get better
maintainability.  Named tuples can be compared with regular tuples.
The json serializer still uses an array to represent it so the on-wire
representation of the object has not changed.  A short snip of code
was required to restore the namedtuple from the RPC message.

Change-Id: I7f8c93b0e12ee0179bb23dfbb3a3d814615b1c2e
Closes-Bug: #1352801
Prevent an iteration through ports on IPv6 slaac

A recent change[1] allowed subnets to be deleted even if they
had active IPv6 SLAAC allocations on them. The updated check
was inefficient because it would check every port in the subnet
even if the subnet was a SLAAC subnet. This patch just shortcuts
out that check.

1. I281f5a1553248e09174dc49d0a42aef4b5c44bee

Change-Id: I2c35495b3642c644e4758f28ccddcc076139ec3b
Enable default SNAT from networks connected to a router indirectly

Make outgoing packets to an external interface SNATed regardless
of source address of the packets. As a result of deep review,
any problem was not found with this change.

Change-Id: I71a1288633bb6af2951d571540bbb9ec5e1270e2
Closes-bug: #1386041
Merge "Enable default SNAT from networks connected to a router indirectly"
Merge "Refactor l2_pop code to pass mac/ip info more readably"
Merge "Prevent an iteration through ports on IPv6 slaac"