This commit enforces a limit on `RawMapper` so that it will not
produce more values than are specified by the LIMIT clause.
Previously the mapper would read up to the chunk size and the
values would be limited afterward.
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.
Ben Johnson
Philip O'Toole
liang@qiniu.com
Nathaniel Cook
Philip O'Toole
mengjingleli
Cory LaNou