MEDIUM: ring: add new srv statement to support octet counting forward

MEDIUM: ring: add new srv statement to support octet counting forward

log-proto <logproto>
  The "log-proto" specifies the protocol used to forward event messages to
  a server configured in a ring section. Possible values are "legacy"
  and "octet-count" corresponding respectively to "Non-transparent-framing"
  and "Octet counting" in rfc6587. "legacy" is the default.

Notes: a separated io_handler was created to avoid per messages test
and to prepare code to set different log protocols such as
request- response based ones.