Rename parallel downloads flags

[sethiay]

Description

Rename parallel downloads flags to:

enable-parallel-downloads
parallel-downloads-per-file
max-parallel-downloads
download-chunk-size-mb
enable-crc

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[bot]]

Codecov Report

Attention: Patch coverage is 67.74194% with 10 lines in your changes missing coverage. Please review.

Project coverage is 72.02%. Comparing base (e247b84) to head (62438a3). Report is 3 commits behind head on master.

Files	Patch %	Lines
cfg/config.go	0.00%	10 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2064 +/- ## ========================================== + Coverage 71.98% 72.02% +0.04% ========================================== Files 99 99 Lines 10869 10879 +10 ========================================== + Hits 7824 7836 +12 + Misses 2713 2710 -3 - Partials 332 333 +1 ``` | [Flag](https://app.codecov.io/gh/GoogleCloudPlatform/gcsfuse/pull/2064/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/GoogleCloudPlatform/gcsfuse/pull/2064/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `72.02% <67.74%> (+0.04%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform#carryforward-flags-in-the-pull-request-comment) to find out more.

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