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
`-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
