EC rebuilds don't use handoff frags

[clayg]

Fragment rebuilds in the EC reconstructor don't take advantage of fragments on handoffs when they can't find enough fragments on primaries.

A partial fix for this bug might improve telemetry for how often this causes a drop in durability. Currently we only emit a log message, e.g.

    object-reconstructor: Unable to get enough responses (1/4) to reconstruct 192.168.190.222:6003/d2/21425/AUTH_demo/ec_container/rc.local policy#2 frag#5 with ETag b9549b2b2ca3627d9f9b108a84db42b2

Unfortunately, this message currently normally means orphan fragment [1]

FWIW, the proxy is a good bit more robust in it's fragment gathering in service of a request. It might be possible to generalize some of that code...

1. https://bugs.launchpad.net/swift/+bug/1655608