Move metric validation and row key generation to TSUID

This is not a good permanent solution. It's just a straight extraction of a few methods to make them reachable on a few parts we're working on. We're working on some stuff in the package ` net.opentsdb.storage` and we need access to this there which we don't currently have.

One solution to this would be to make `net.opentsdb.core.IncomingDataPoints` and the methods public but we're refactoring some other stuff there and that probably won't be merged for a while. Thus why we want to get this stuff out of there and merged so we can try to avoid some conflicts...