CDS: switch persistence to persisting the candidates

CDS: switch persistence to persisting the candidates

With us being in control of the DataTree, we can now use
DataTreeCandidate for replication and persistence. Add the appropriate
payload and add appropriate hooks.

Exposing the DataTreeCandidate from the cohort is actually pushing
towards having our own cohort type -- ShardDataTreeCohort. With that we
can get rid of DOMStoreThreePhaseCommitCohort, so eliminate its mention
to prevent accidental leakage.

Change-Id: I37a22b445f955330193c762894764feee94bebdb
Signed-off-by: Robert Varga <rovarga@cisco.com>