DTable reduce (on grouped dtable) performance improvements

[krynju]

(merge before the tables.jl pr - will need to adjust that one once this is merged)

Noticed the grouped reduce is super slow for no real reason (normal reduce is super quick), remembered I just wrapped over normal reduce.
Was a good idea, but it was allocating a lot more than it should and just doing unnecessary work which then had to be transformed later anyway.

# THIS PR
# https://github.com/JuliaParallel/Dagger.jl/pull/294

PS C:\Users\krynjupc\.julia\dev\Dagger> julia -t16 .\dtable_bench2.jl
first groupby (compile overhead)
 46.232115 seconds (291.47 M allocations: 17.844 GiB, 37.25% gc time, 20.10% compilation time)
second groupby
 46.863056 seconds (278.45 M allocations: 17.126 GiB, 50.37% gc time)
first reduce (compile included)
 17.210092 seconds (41.67 M allocations: 2.415 GiB, 54.87% gc time, 17.93% compilation time)
second reduce
 11.101848 seconds (32.15 M allocations: 1.903 GiB, 57.17% gc time)
groupby reduce
 99.229038 seconds (330.47 M allocations: 20.138 GiB, 68.62% gc time)

# CURRENT MASTER
# https://github.com/JuliaParallel/Dagger.jl/pull/291

PS C:\Users\krynjupc\.julia\dev\Dagger> julia -t16 .\dtable_bench2.jl
first groupby (compile overhead)
 43.010412 seconds (294.32 M allocations: 18.008 GiB, 38.43% gc time, 23.85% compilation time)
second groupby
 45.847915 seconds (278.09 M allocations: 17.156 GiB, 49.82% gc time)
first reduce (compile included)
 58.989759 seconds (136.16 M allocations: 8.198 GiB, 60.60% gc time, 14.50% compilation time)
second reduce
 71.114289 seconds (156.93 M allocations: 8.890 GiB, 58.00% gc time)
groupby reduce
227.729765 seconds (575.97 M allocations: 32.183 GiB, 64.63% gc time, 0.08% compilation time)

[codecov-commenter]

Codecov Report

Merging #294 (4229f91) into master (defe1b1) will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff           @@
##           master    #294   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files          39      39           
  Lines        3184    3189    +5     
======================================
- Misses       3184    3189    +5     

Impacted Files	Coverage Δ
src/table/operations.jl	0.00% <0.00%> (ø)

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[krynju]

@jpsamaroo This is good to go now. Just added some thing to better resolve columnnames - need a vector of symbols there in the, so had to add some additional code to handle that

[jpsamaroo]

Thanks!