Use Parse::Syslog::Line for parsing of syslog messages in Input::Syslog
Remove remote_hostname attribute in Input::Syslog

because RFC5426 states in section 3.4:
The source IP address of the UDP datagrams SHOULD NOT be interpreted
as the identifier for the host that originated the syslog message.

Instead the IPv4 or IPv6 address is always stored in the received_from field