Add a OpenTSDB input protocol

This adds an input supporting the telnet put interface from OpenTSDB.
OpenTSDB's tag keys and values and tranformed into Inlflux DB columns.
At present all values are stored as 64 bit floats.

This is based on a mix of the UDP, and Graphite APIs. I've given it a bit of initial testing with tcollector, and so far things looks OK. Not load tested it yet.

I noticed the graphite collector uses a buffered channel, where the UDP one does not. Is the buffering taken care of elsewhere now? TSDB clients will be quite bursty, so buffering may improve things if needed