Shard Mappers perform query re-writing

[otoolep]

With this change query-rewriting, critically wildcard-rewriting, is performed by the LocalMappers and not by Executors. This is necessary since only shard indexes know what fields should replace * within a given SELECT query.

This required the following changes:

• Mapper responses now include the list of fields names, always in alphabetical order, for the data being returned to the Executor.
• The Executor, when in raw mode, can use this to form the complete set of column names.
• The Executor does not now re-write queries, but still uses the version of the statement passed to it to initialize reducer functions (in the case of aggregate queries) and mathematical functions. Fortunately this processing does not require wildcards or regexes to be replaced so the Executor can continue to work with the non-rewritten version of the SELECT statement.
• Aggregate queries did not need much work, since aggregate queries may not contain wildcards, so re-writing is irrelevant.
• RemoteMapper updated as needed.
• Unit tests updated as needed. An important new test was added -- SELECT * FROM cpu where the query requires that two complete decoupled shards (and TSDB stores) objects mush be processed.

[otoolep]

@pauldix @dgnorton