frollapply throttle usage of multiple threads

[jangorecki]

closes #7305

using same mechanism as we use for openmp throttle.
verbose nicely reports all details.

also updated timings in manual, which were 3 years old.

library(data.table)

x = sample(1e3, 2)
n = 1L
f = mean

setDTthreads(1)
system.time(frollapply(x, n, f, simplify=unlist))
#   user  system elapsed 
#  0.001   0.000   0.001
setDTthreads(2) ## more than 2 would not be used anyway, because length(x)==2
system.time(frollapply(x, n, f, simplify=unlist))
#   user  system elapsed 
#  0.001   0.000   0.001

setDTthreads(1)
system.time(for (i in 1:1e3) frollapply(x, n, f, simplify=unlist))
#   user  system elapsed 
#  0.098   0.002   0.100
setDTthreads(2)
system.time(for (i in 1:1e3) frollapply(x, n, f, simplify=unlist))
#   user  system elapsed 
#  0.088   0.001   0.089 

[github-actions]

No obvious timing issues in HEAD=frollapply-throttle

Generated via commit 4ea30ea

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	3 minutes and 0 seconds
Installing different package versions	43 seconds
Running and plotting the test cases	3 minutes and 1 seconds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (766efde) to head (4ea30ea).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7306   +/-   ##
=======================================
  Coverage   99.08%   99.08%           
=======================================
  Files          83       83           
  Lines       15802    15814   +12     
=======================================
+ Hits        15657    15669   +12     
  Misses        145      145           

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