Add support for RequestMetadata

These changes introduce support for sending and receiving
`RequestMetadata` header values that can be attached to RPC calls.

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.)

The `bgd execute` command now can send `RequestMetadata` values,
which can be specified through CLI parameters.