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.

docs/main.qbk

@@ -1,5 +1,5 @@
 [/
-    Copyright (c) Copyright (c) 2012, 2013 Ripple Labs Inc.
+    Copyright (c) Copyright (c) 2012-2017 Ripple Labs Inc.
 
     Permission to use, copy, modify, and/or distribute this software for any
     purpose  with  or without fee is hereby granted, provided that the above
@@ -16,7 +16,7 @@
 
 [library rippled
     [quickbook 1.6]
-    [copyright 2012 - 2016 Ripple Labs Inc.]
+    [copyright 2012 - 2017 Ripple Labs Inc.]
     [purpose C++ Library]
     [license
         Distributed under the ISC License
@@ -30,6 +30,8 @@
 [template indexterm1[term1] '''<indexterm><primary>'''[term1]'''</primary></indexterm>''']
 [template indexterm2[term1 term2] '''<indexterm><primary>'''[term1]'''</primary><secondary>'''[term2]'''</secondary></indexterm>''']
 
+[include consensus.qbk]
+
 [section:ref Reference]
 [include temp/reference.qbk]
 [endsect]