BGP ribs: do not merge routes with unreachable next hops

[progwriter]

Because that's how BGP actually works.

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/irg-xe-3s-book/bgp-support-for-next-hop-address-tracking.html

[batfish-bot]

This change is 

[codecov]

Codecov Report

Merging #6274 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #6274      +/-   ##
============================================
- Coverage     72.91%   72.90%   -0.01%     
- Complexity    35015    35026      +11     
============================================
  Files          2832     2832              
  Lines        142275   142324      +49     
  Branches      17085    17094       +9     
============================================
+ Hits         103741   103766      +25     
- Misses        30313    30328      +15     
- Partials       8221     8230       +9     

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/org/batfish/dataplane/rib/Bgpv4Rib.java	100.00% <100.00%> (ø)	7.00 <6.00> (+6.00)
...main/java/org/batfish/datamodel/PrefixIpSpace.java	87.50% <0.00%> (-5.84%)	8.00% <0.00%> (ø%)
.../org/batfish/dataplane/rib/RouteAdvertisement.java	83.67% <0.00%> (-4.09%)	18.00% <0.00%> (-1.00%)
...c/main/java/org/batfish/dataplane/rib/RibTree.java	88.33% <0.00%> (-1.67%)	28.00% <0.00%> (-1.00%)
...col/src/main/java/org/batfish/role/InferRoles.java	89.54% <0.00%> (-1.37%)	50.00% <0.00%> (-1.00%)
...src/main/java/org/batfish/coordinator/PoolMgr.java	59.25% <0.00%> (-1.24%)	15.00% <0.00%> (-1.00%)
...java/org/batfish/dataplane/ibdp/VirtualRouter.java	90.47% <0.00%> (-0.33%)	208.00% <0.00%> (-1.00%)
...src/main/java/org/batfish/coordinator/WorkMgr.java	75.85% <0.00%> (-0.32%)	246.00% <0.00%> (-1.00%)
...tfish/representation/cisco/CiscoConfiguration.java	85.87% <0.00%> (-0.14%)	492.00% <0.00%> (-1.00%)
...fish/representation/cisco/CiscoStructureUsage.java	100.00% <0.00%> (ø)	3.00% <0.00%> (ø%)
... and 7 more

[anothermattbrown]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved