Requiring 2/2 backends for PUT requests means that the cluster can't
tolerate a single failure. Likewise, if you have 4 replicas in 2
regions, requiring 3/4 on a POST request means you cannot POST with
your inter-region link down or congested.
This changes the (replication) quorum size in the proxy to be at least
half the nodes instead of a majority of the nodes.
Daemons that were looking for a majority remain unchanged. The
container reconciler, replicator, and updater still require majorities
so their functioning is unchanged.
Odd numbers of replicas are unaffected by this commit.
Change-Id: I3b07ff0222aba6293ad7d60afe1747acafbe6ce4
29544a9e17