While investigating the reported hostcache
issues and adding a histogram to look at the
distribution of used hashbuckets, and
bucketdepth, I found that the exteremly
fast XOR "hash" would result in a rather
skewed utilization of the hostcache, for
randomly selected remote IP addresses.

As other parts of the base tcp stack (eg.
tcp fastopen) already use jenkins_hash32,
and the properties appear reasonably good,
switching to use that.

Anecdotal evidence (single trial, using
approximately 1000 randomly selected hosts,
overall, but ~400 for XOR and ~600 for Jenkins):

Unused Hash buckets: XOR ~51%, Jenkins ~35%
Average Bucket Depth: XOR ~2.44, Jenkins ~1.54