Local read affinity for GET/HEAD requests.

Now you can configure the proxy server to read from "local" primary
nodes first, where "local" is governed by the newly-introduced
"read_affinity" setting in the proxy config. This is desirable when
the network links between regions/zones are of varying capacities; in
such a case, it's a good idea to prefer fetching data from closer
backends.

The new setting looks like rN[zM]=P, where N is the region number, M
is the optional zone number, and P is the priority. Multiple values
can be specified by separating them with commas. The priority for
nodes that don't match anything is a very large number, so they'll
sort last.

This only affects the ordering of the primary nodes; it doesn't affect
handoffs at all. Further, while the primary nodes are reordered for
all requests, it only matters for GET/HEAD requests since handling the
other verbs ends up making concurrent requests to *all* the primary
nodes, so ordering is irrelevant.

Note that the default proxy config does not have this setting turned
on, so the default configuration's behavior is unaffected.

blueprint multi-region

Change-Id: Iea4cd367ed37fe5ee69b63234541d358d29963a4
Merge "Local read affinity for GET/HEAD requests."