Allow for retrieving TSMeta by metric name and tags.

We need to retrieve TSMeta objects by metric name and tags, since we plan to use TSMeta to store different configurations. Since timeseries might be added dynamically, we need to be able to retrieve TSMeta objects just by metric name and tags, since we do not necessarily know the TSUID at that point in time.

I added a new parameter to the "/api/uid/tsmeta" endpoint, that can be used as an alternative to "tsuid=...". The format is "m=metric{tagk1=tagv1,...}", so basically the query format without aggregators. It's available at https://github.com/kodemaniak/opentsdb/tree/next.

A metric name and tags combination has to be specified that points to an existing timeseries. No aggregation is performed here, though returning several matching TSMeta objects could be an option. A 404 HTTP status is returned, when tags are incomplete or the timeseries does not exist.