Fix a performance regression with get_socket_config()

When reloading config that has a gigantic number of sockets,
running get_socket_config() is very expensive. Actually it's
O(n**2). To fix that we create a temporary dictionary, with
key as socket name and reduce complexity to O(1).
Cosmetic: if one section name matches, don't check others

And also, warn when we read bad section
Add a rough hack: support "socket_file:" section

The problem with config loading is that it's terribly complex and
slow. For loading socket configs we don't need all the complexity,
just want to load the basic flat file.

To enable that we introduce a proper hack: socket_file section,
which takes a single parameter: filename. Circus will load this
file and all socket: sections from it. No validation is done. This
is intended to be fast.