Previously, seamless reloads were a little risky: when they worked, they
worked great, but if they failed (say, because you wrote out an invalid
config), you were left with no usable server processes and possible
client downtime.
Now, add the ability to do a preflight check before reloading processes
to reduce the likelihood of the reloaded process immediately dying. For
example, you might use a systemd unit that includes something like
ExecReload=swift-proxy-server --test-config /etc/swift/proxy-server.conf
ExecReload=kill -USR1 $MAINPID"
Change-Id: I9e5e158ce8be92535430b9cabf040063f5188bf4
0a5f0253b1