cluster-id length equality for multipath

A fat tree topology running IBGP gets into two issues with anycast address
routing. Consider the following topology:

        R9   R10
          x x
  R3   R4     R7   R8
     x           x
  R1   R2     R5   R6
  |    |      |    |
 10/8 10/8  10/8   S

Let's remind ourselves of BGP decision process steps:

1. Highest Local Preference
2. Shortest AS Path Length
3. Lowest Origin Type
4. Lowest MED (Multi-Exit Discriminator)
5. Prefer External to Internal
6. Closest Egress (Lowest IGP Distance)
7. Tie Breaking (Lowest-Router-ID)
8. Tie Breaking (Lowest-cluster-list length)
9. Tie Breaking (Lowest-neighbor-address)

Without any policies, steps 1-6 will almost always evaluate identically for
all paths received on any router in the above topology. Let's assume that
the router-ids follow the following inequality: R1 < R2 < R5 < R6. Owing to
the 7th step above, all routers will now choose R1's path as the best. This
is undesirable. As an example, traffic from S to 10/8 will follow the path
S -> R6 -> R7 -> R9 -> R4 -> R2 -> 10/8 instead of S -> R6 -> R7 -> R5 -> 10/8.
Furthermore, once R7 (& R8) chooses R1's path as the best, it would withdraw
its path learned through (R5, R6) from (R9, R10). This leads to inefficient
load balancing - e.g. R9 can't do ECMP across all available egresses -
(R1, R2, R5).

The patch addresses these issues by noting that that cluster list is always
carried along with the routes and its length is a good indicator of IBGP
hops. It thus makes sense to compare that as an extension to metric after
step 6. That automatically ensures correct multipath computation.

Unfortunately a partial deployment of this in a generic topology (note:
fat-tree/clos topologies work fine) may lead to potential loops. It needs
to be looked into.

Signed-off-by: Pradosh Mohapatra <pmohapat@cumulusnetworks.com>
Reviewed-by: Dinesh G Dutt <ddutt@cumulusnetworks.com>

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