[ofa-general] removing PR caching from IPoIB

[Sean Hefty]

> What is the motivation for these changes?  If you explain what you're
> trying to fix, then maybe I can give a more intelligent answer.

IPoIB permanently caches PRs with no mechanism in place to keep them in 
sync with the SA.  The goal is to limit the PR lifetime to prevent the 
use of stale paths.

A side benefit of limiting the PR scope to a neighbour is that different 
neighbours could use different paths, versus using a single path per DGID.

- Sean