Proxy should hold requests while building.

[brandonbloom]

Presently, the retry loop is hard coded to 10 attempts with 100ms between attempts. Even without accounting for some of the configurable delays, builds can exceed 1 second. In that case, the attempt limit will always be exceeded.

Ideally, the proxy server should buffer requests while building, so that responses to clients are delayed until building finishes instead of being forced to retry. This would result in a better developer experience, since frequently the leaf of the retry tree is a human pressing refresh repeatedly in their browser. I'd rather hit refresh and wait a few seconds, than wait a few seconds while refreshing repeatedly.