MAJOR: log: introduce log backends

MAJOR: log: introduce log backends

Using "mode log" in a backend section turns the proxy in a log backend
which can be used to log-balance logs between multiple log targets
(udp or tcp servers)

log backends can be used as regular log targets using the log directive
with "backend@be_name" prefix, like so:

  | log backend@mybackend local0

A log backend will distribute log messages to servers according to the
log load-balancing algorithm that can be set using the "log-balance"
option from the log backend section. For now, only the roundrobin
algorithm is supported and set by default.