Use default MaxDownloadParallelism based on number of CPU cores

[sethiay]

Description

Use default MaxDownloadParallelism based on number of CPU cores. Also set a minimum value of 16 in case the cores of CPU is less (2 or 4).

Using default 2 * number of cores as with the testing so far with a workload, this value looks good.

And couple of more things to tell upfront :

1. This is based on CPU cores count of machine, so on GKE this will translate to machine's CPU cores and not on the CPU limit on pod. We are fine here because pod limit <= machine CPU cores. So, we are just assigning higher value to max parallel downloads there.
2. It's possible to mount multiple GCSFuse mounts on single GCE VM/GKE node. In that case, one GCSFuse default is not affected by the other.

Link to the issue in case of a bug fix.

NA

Testing details

1. Manual - NA
2. Unit tests - NA
3. Integration tests - NA

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.94%. Comparing base (dd1542d) to head (42783e6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2047      +/-   ##
==========================================
- Coverage   71.95%   71.94%   -0.02%     
==========================================
  Files          99       99              
  Lines       10870    10872       +2     
==========================================
  Hits         7822     7822              
- Misses       2715     2717       +2     
  Partials      333      333              

Flag	Coverage Δ
unittests	71.94% <100.00%> (-0.02%)	⬇️

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[kislaykishore]

@sethiay could you update the params.yaml as well? You'll need to make the following changes:

• Add "runtime" here
• Update the default here to "int64(math.Max(16, 2*runtime.NumCPU()))". Alternatively, you can create a function in cfg/ and reference it here.
• Regenerate the config.go by calling go generate ./...

[sethiay]

@sethiay could you update the params.yaml as well? You'll need to make the following changes:

• Add "runtime" here
• Update the default here to "int64(math.Max(16, 2*runtime.NumCPU()))". Alternatively, you can create a function in cfg/ and reference it here.
• Regenerate the config.go by calling go generate ./...

As discussed offline, we will handle this in a separate PR.