Add timeouts for socket operations. ssbench-master now takes two new
arguments, --connect-timeout <float_seconds> and --network-timeout
<float_seconds>. The first is a timeout for connecting to the Swift
proxy (or load-balancer or whatever), and the second timeout applies to
all socket operations after the connect. They default to 10.0 and 20.0
Also fix bug where the storage_url would get overridden with None.