Now all gRPC calls go through the `RequestMetadataInterceptor`, which
appends the relevant values that were set on the channel creation.

The CLI commands now have options that allow the user to set any of the
RequestMetadata fields to custom values.

The `Scheduler` now keeps track of that information and could employ
them for logging and monitoring purposes when a remote session is
finished.

This involved some refactoring and the introduction of a new
`Peer` class, which also keeps track of all the `Peer` instances
(indexed by `Peer.uid` values).