Improve Consensus interface and documentation (RIPD-1340):

- Add Consensus::Result, which represents the result of the
establish state and includes the consensus transaction set, final
proposed position and disputes.
- Add Consensus::Mode to track how we are participating in
consensus and ensures the onAccept callback can distinguish when
we entered the round with consensus versus when we recovered from
a wrong ledger during a round.
- Rename Consensus::Phase to Consensus::State and eliminate the
processing phase.  Instead, accept is a terminal phase which
notifies RCLConsensus via onAccept callbacks.  Even if clients
dispatch accepting to another thread, all future calls except to
startRound will not change the state of consensus.
- Move validate_ status from Consensus to RCLConsensus, since
generic implementation does not directly reference whether a node
is validating or not.
- Eliminate gotTxSetInternal and handle externally received
TxSets distinct from locally generated positions.
- Change ConsensusProposal::changePosition to always update the
internal close time and position even if we have bowed out. This
enforces the invariant that our proposal's position always
matches our transaction set.

CMakeLists.txt

@@ -353,6 +353,7 @@ if (WIN32 OR is_xcode)
     docs/
     Jamfile.v2
     boostbook.dtd
+    consensus.qbk
     index.xml
     main.qbk
     quickref.xml