Adding tls-tcp mode to syslog output.

This patch lets you send syslog messages from logstash using TLS over TCP. This is the preferred way to send syslog over TCP ([RFC 5424](https://tools.ietf.org/html/rfc5424), 5.1 says "All implementations of this specification MUST support a TLS-based transport as described in [RFC5425].").

I tested this by forwarding messages to Papertrail (configured to only use TLS over TCP) from a logstash agent with 'tls-tcp' syslog configured.