Handle multiple connections simultaneously

Handle multiple connections simultaneously

This commit changes the implementationt to store connected clients
file-descriptors and to iterate through them all periodically to handle
requests and to time-out stale connections. This means that even if one
connection is in progress, the program can still handle new connections
from other clients.

Note this does this not by threads but by taking advantage of
non-blocking io to handle each connection.

Fixes https://git.seodisparate.com/stephenseo/c_simple_http/issues/8 .