concurrent stat requests when listing shares

[butonic]

The sharesstorageprovider now concurrently stats the accepted shares when listing the share jail. The default number of 5 workers can be changed by setting the max_concurrency value in the config map.

fixes owncloud/ocis#9828

with 500 accepted shares on a local machine:

➜  reva git:(edge) http --meta --verify=no -a einstein:relativity PROPFIND https://cloud.ocis.test/dav/spaces/a0ca6a90-a365-4782-871e-d44447bbc668 Depth:1
Elapsed time: 0.252219335s

➜  reva git:(sharesstorageprovider-concurrent-stat) http --meta --verify=no -a einstein:relativity PROPFIND https://cloud.ocis.test/dav/spaces/a0ca6a90-a365-4782-871e-d44447bbc668 Depth:1
Elapsed time: 0.0148590878s

and on a kubernetes deployment with 200 shares

➜  reva git:(edge) http --meta --verify=no -A bearer -a jwt.data.hash PROPFIND https://cloud.ocis.test/dav/spaces/a0ca6a90-a365-4782-871e-d44447bbc668 Depth:1
Elapsed time: 2.7347017197s

➜  reva git:(sharesstorageprovider-concurrent-stat) http --meta --verify=no -A bearer -a jwt.data.hash  PROPFIND https://cloud.ocis.test/dav/spaces/a0ca6a90-a365-4782-871e-d44447bbc668 Depth:1
Elapsed time: 0.9289751261s