Allow configuration of upload concurrency level

## Is your feature request related to a problem? Please describe.
Some S3 implementation need a certain level of concurrency during MultiPartUploads to offer the maximum bandwidth. Currently the concurrency is fixed 4 parallel threads in oCIS.

## Describe the solution you'd like
To be able to tweak the level of concurrency to the amount needed for a certain s3 implementation the fixed value of 4 parallel threads should be adjustable.

## Describe alternatives you've considered
None

## Additional context
The same issue was addressed in oC10 in https://github.com/owncloud/files_primary_s3/pull/605. So this feature is also meant to achieve feature parity.