Instead of using bool flag, just stop prepare watcher.

This is a legacy of SPDY era where it can disable flow control.

This is the unit of sharing configurations to change

For those connections via frontend with api parameter, they use solely
APIDownstreamConnection.

In this commit, APIDownstreamConnection just consumes all request
body, and do nothing.  The next few commits implements our first API
endpoint: /v1/api/dynamicconfig.

fix warning: declaration of 'free' shadows a global declaration

With some old OS X release

Write PID in temporary file first.  Then rename it as the real
destination.  It will avoid the issue that the external process may
read the empty PID file because of race condition.