Add JoinQuery

[jihoonson]

Second patch for #4032.

Here are the highlights of the changes.

• Extended Query to be able to have multiple data sources.
• Added JoinQuery
• Added a new method annotateDistributionTarget to QueryToolChest. CachingClusteredClient can figure out which data source is the target of query distribution when choosing nodes for query processing.
• DimensionSpec is changed to have an optional dataSource name field. Another option for this is to force the name of DimensionSpec to always be prefixed with its dataSource name like 'foo.dim1'. I think this is error-prone, and thus the former option is better.
• Currently, metrics are represented as a list of simple strings, so they must be forced to be prefixed their dataSource names. However, in the future, I think we need a new data structure for metrics like DimensionSpec, or extend DimensionSpec to cover metrics as well. (It's possible because it now supports long and double columns.)

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