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• Updated associated README.md.
• Wrote appropriate unit tests.
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This PR resolves #9034.

It adds support for TCP. Servers (endpoints) may now be specified as URLs with either udp or tcp scheme, eg. "udp://127.0.0.1:12201" or "tcp://127.0.0.1:12201". For compatiblity with existing configurations, if scheme is missing (eg. "127.0.0.1:12201") or is not udp neither tcp, UDP protocol is assumed.

Sample config has been changed to reflect it:

  ## Endpoints for your graylog instances.
  servers = ["udp://127.0.0.1:12201"]

  ## Connection timeout.
  # timeout = "5s"

  ## The field to use as the GELF short_message, if unset the static string
  ## "telegraf" will be used.
  ##   example: short_message_field = "message"
  # short_message_field = ""

Config option timeout allows to change connection dialup timeout.

Connection handling has been refactored. Although I'm not quite sure, it may be OK for UDP to have a connection created for each outgoing message, but it would not work well for TCP where network resources would get depleted quickly. Therefore connections are reused (and recreated whenever needed, ie. typically after write error occurs).