Unify uploads handling under filestore package

Base part from splitting !227 (merged) into multiple MRs.

Here we are introducing the filestore package that will handle file uploads to shared disk and Object Storage, by now only shared disk is implemented.

This MR is a prerequisite for gitlab-org/gitlab-ee#4184

Although extending the information provided by workhorse to rails (file size and multiple hashes), this patch does not require any change in GitLab API.

/cc @ayufan