MEDIUM: server: Refactor apply_server_state() to make it more readable

MEDIUM: server: Refactor apply_server_state() to make it more readable

The apply_server_state() function is really hard to read. Thus it was
refactored to be more maintainable. First, an helper function is used to get
the server-state file path. Some useless variables were removed and most of
other variables were renamed to be more readable. The error messages are now
prefixed to know the context (global vs per-proxy). Finally, the loop on the
proxies list was simplified.

This patch may seem a bit huge, but the changes are not so important.