This commit refactors the tsdb query engine to use separate aggregate
and raw execution paths, encapsulates cursor functionality, and removes
the TagSetCursor from the aggregate path. By removing the TagSetCursor,
we can pass sets of unordered values to the map functions and bypass
the `container/heap` entirely.
This commit adds a cursor that wraps multiple `tsdb.Cursor` objects
and streams them out as one cursor. The multi-cursor automatically
dedupes keys by using the first cursor specified in the argument
list.
Cory LaNou
Ben Johnson
Jason Wilder