[DOP-7051] Support parallel operations in file classes

[dolfinus]

Change Summary

Uploading/downloading/moving files one by one maybe OK for small number of files, but definitely not for many. For example, uploading 1k files to HDFS takes about 13 minutes - namenode ensures that file blocks are replicated to proper number of data nodes, and responses to client only after that, and the rest of files are waiting for that.

All these operations are IO-bound tasks, so Python releases GIL, and we can use threads to run them in parallel.

That was changed:

• Internal implementation of FileMover/FileDownloader/FileUploader was enhanced to use ThreadPoolExecutor for parallel file operations.
• FileMover.Options / FileDownloader.Options / FileUploader.Options classes got new option workers: int which can be set by user to switch between plain old for loop and ThreadPoolExecutor with specific number of workers.
• Internal implementation of FileConnection.client and related _get_client/_is_client_closed/_close_client was updated to create and cache a separated client for each thread, avoiding issues with using non-thread safe client implementations (most of them are not safe).
• Skipped duplicated check for source_file/local_file existance - it was performed both before and during file handling process.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

Merging #57 (0be9631) into develop (0210d99) will increase coverage by 0.05%.
The diff coverage is 97.76%.

@@             Coverage Diff             @@
##           develop      #57      +/-   ##
===========================================
+ Coverage    92.70%   92.75%   +0.05%     
===========================================
  Files          126      126              
  Lines         6000     6101     +101     
  Branches      1109     1144      +35     
===========================================
+ Hits          5562     5659      +97     
  Misses         345      345              
- Partials        93       97       +4     

Impacted Files	Coverage Δ
...netl/connection/file_connection/file_connection.py	93.68% <94.11%> (-0.18%)	⬇️
onetl/file/file_mover/file_mover.py	97.19% <97.61%> (-0.15%)	⬇️
onetl/file/file_uploader/file_uploader.py	94.81% <97.87%> (+0.39%)	⬆️
onetl/file/file_downloader/file_downloader.py	94.93% <97.91%> (+0.23%)	⬆️
onetl/connection/file_connection/ftp.py	96.34% <100.00%> (ø)
onetl/connection/file_connection/hdfs.py	95.92% <100.00%> (ø)
onetl/connection/file_connection/s3.py	97.12% <100.00%> (ø)
onetl/connection/file_connection/sftp.py	86.29% <100.00%> (ø)
onetl/connection/file_connection/webdav.py	97.27% <100.00%> (ø)
onetl/log.py	85.71% <100.00%> (ø)