allow showProgress=INTEGER to set progress bar update time

[ben-schwen]

Closes #6514

# simple example to trigger progress bar
f = function(x) {Sys.sleep(1); x}
dt = data.table(x = 1:10, y = 1:10)
dt[, f(x), y, showProgress=3L]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (851467f) to head (ee671af).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7259      +/-   ##
==========================================
+ Coverage   98.96%   98.99%   +0.03%     
==========================================
  Files          87       87              
  Lines       16734    16729       -5     
==========================================
+ Hits        16560    16561       +1     
+ Misses        174      168       -6     

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[github-actions]

• HEAD=showProgress_update stopped early for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit ee671af

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

Task	Duration
R setup and installing dependencies	2 minutes and 49 seconds
Installing different package versions	44 seconds
Running and plotting the test cases	4 minutes and 26 seconds

[tdhock]

Thanks for the PR Ben. Is there a reason why the delay is integer rather than double/numeric? (which seems more natural for a time)

[ben-schwen]

Thanks for the PR Ben. Is there a reason why the delay is integer rather than double/numeric? (which seems more natural for a time)

I also thought about using doubles first, but then integer seemed like the right choice since everything below 1/4 second is hard to grasp so the granularity seems to be coarse enough and I also didn't want to cater for strange supplied values like NA (behaves like -INT_MAX), Inf, etc.

[MichaelChirico]

LGTM, feel free to merge after the quick clean-ups requested. Thanks!