After the initial requests are started, if the proxy still does not have
enough backend responses to return a client response additional requests
will be spawned to remaining primaries at the frequency configured by
the concurrency_timeout.
A new tunable concurrent_ec_extra_requests allows operators to control
how many requests to backend fragments are started immediately with a
client request to an object stored in an EC storage policy. By default
the minimum ndata backend requests are started immediately, but
operators may increase concurrent_ec_extra_requests up to nparity which
is similar in effect to a concurrency_timeout of 0.
Change-Id: Ia0a9398107a400815be2e0097b1b8e76336a0253
8f60e0a260