Closes gitaly#929

Closes gitaly#851

Closes gitaly#863

Closes gitaly#866

Due to the refactoring in !16082, `Blob#batch` no longer falls back
to a default `blob_size_limit`. Since `Repository#batch_blobs` was using
a default `nil` value, this would cause issues in the `Blob#find_by_rugged`
method.

This fix here is to be consistent and use a non-nil default value in
`Repository#batch_blobs`.

The problem was masked in development and tests because Gitaly is always
enabled by default for all features.

Closes #41735

Signed-off-by: Rémy Coutable <remy@rymai.me>

This reverts merge request !15712

By importing this Ruby code into gitlab-rails (and gitaly-ruby), we avoid
200ms of startup time for each gitlab_projects subprocess we are eliminating.

By not having a gitlab_projects subprocess between gitlab-rails / sidekiq and
any git subprocesses (e.g. for fork_project, fetch_remote, etc, calls), we can
also manage these git processes more cleanly, and avoid sending SIGKILL to them

Closes gitaly#808

This also allows us to simplify the naming since we can make some
fetching methods private.

Closes gitaly#780

Closes gitaly#737