Enhancement: Allow limiting IO concurrency for local and SFTP backend

Restic did not support limiting the IO concurrency / number of connections for
accessing repositories stored using the local or SFTP backends. The number of
connections is now limited as for other backends, and can be configured via the
the `-o local.connections=2` and `-o sftp.connections=5` options. This ensures
that restic does not overwhelm the backend with concurrent IO operations.

https://github.com/restic/restic/pull/3475
