Added support for binding-independent RPCs

Implemented the support for binding-independent RPCs directly into BrokerImpl,
and removed unused placeholders for functionality.

Cleanup:
  - Moved Session Implementations directly into BrokerImpl as private inner
    classes, because they are BrokerImpl specific.
  - Removed unnecessary packages.
  - Extracted RPC utils from sal-broker-impl to sal-common-utils to be reusable
    for other components without introducing dependency on sal-broker-impl.

Signed-off-by: Tony Tkacik <ttkacik@cisco.com>