Series collapse and joined queries

Implements collapsing a series of data into a single timestamp, allowing easy computation across a time range.  For example, "maximum value in last 7 days".
Implements joining multiple queries in both `get()` and `series()` by passing in a list of timeseries names instead of a single string. Allows for more powerful queries, and combined with collapsed data, aggregates across many different timeseries. For example, histograms that track instances of unique values that may occur in multiple timeseries can be joined together, collapsed and a count of the total number of unique values in a date range can be calculated in a single call.