write_graphite plugin: UDP support and LogSendErrors config flag

##### Adding TCP/UDP support via config
UDP allows to "fire-and-forget" data while not bringing the servers down due to TCP's connection oriented overhead.

The new versions of Graphite support both TCP and UDP streams so it made sense to add UDP support.

##### Adding LogSendErrors boolean flag
Sometimes we don't want to hog the syslog with errors that are produced when the graphite server is down.

Especially for UDP, we want to be able to truly fire-and-forget (remember to monitor UDP packet receipt failures though :) and having errors written to the syslog is a burden that should be avoided.