connmgr: Modernize coding style.

This moves declarations closer to first use and merges them with
initialization when possible, moves "for" loop variable declarations into
the "for" statements where possible, and otherwise makes this code look
like it was written a little more recently than it was.

Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>