Fix "Handoff requested (N)" log spam

A long, long time ago, on a GET request, the proxy would go look on 3*
nodes for the requested thing. If one of the primary nodes was
error-limited, it'd look on two primaries and a handoff. Since this
indicated some failure somewhere, the proxy would emit a warning:
"Handoff requested (1)". If two primaries were down, there'd be a
second message "Handoff requested (2)", and so on.

Some StatsD messages were emitted too.

A somewhat shorter time ago (commit d79a67eb), the proxy started
looking into handoffs if it got 404s from the primaries. While this
was a good idea, it resulted lots of "Handoff requested (N)" log spam;
you'd see these messages on every single 404. Also, the StatsD
handoff_count and handoff_all_count metrics shot way up and turned
into noise.

This commit restores the original intent (and usefulness) of the log
messages and StatsD metrics: if the proxy only looks at the normal
number of handoff nodes, nothing is logged. However, if a primary is
down, then the message "Handoff requested (1)" will be logged,
indicating that the proxy looked at one more handoff than it normally
would, and this happened because a primary node was error-limited.

Closes-Bug: 1297214

* or whatever the replica count was

Change-Id: If1b77c18c880b096e8ab1df3008db40ce313835d
Merge "Fix "Handoff requested (N)" log spam"