CDS: Introduce ChainedTransactionIdentifier

CDS: Introduce ChainedTransactionIdentifier

Conceptually the identifier of a a transaction is an extension point,
which allows passing of arbitratry, extensible identifier. It can thus
be reused to track the chain identifier as well as the normal source
coordinates.

Introduce ChainedTransactionIdentifier to carry the transaction chain ID
attached to the transaction on the frontend. This is not used in this
commit directly, but rather in the follow-up patch.

Change-Id: Iad24a1b59c6231e003579bb25149b73b11d6db25
Signed-off-by: Robert Varga <rovarga@cisco.com>