switch default write_graphite protocol back to TCP

The *default* protocol was switched to UDP in 498a0dcd, when UDP support
was added to the plugin.

Switching this default back to TCP, because it makes the plugin break
for users upgrading to 5.4, and also because the UDP listener is disabled
in a default graphite installation (cf.
https://github.com/graphite-project/carbon/blob/0.9.x/conf/carbon.conf.example#L78)

Fixes GH#428