Refactor consensus for simulation (RIPD-1011):

This is a substantial refactor of the consensus code and also introduces
a basic consensus simulation and testing framework.  The new generic/templated
version is in src/ripple/consensus and documents the current type requirements.
The version adapted for the RCL is in src/ripple/app/consensus.  The testing
framework is in src/test/csf.

Minor behavioral changes/fixes include:
* Adjust close time offset even when not validating.
* Remove spurious proposing_ = false call at end of handleLCL.
* Remove unused functionality provided by checkLastValidation.
* Separate open and converge time
* Don't send a bow out if we're not proposing
* Prevent consensus stopping if NetworkOPs switches to disconnect mode while
  consensus accepts a ledger
* Prevent a corner case in which Consensus::gotTxSet or Consensus::peerProposal
  has the potential to update internal state while an dispatched accept job is
  running.
* Distinguish external and internal calls to startNewRound.  Only external
  calls can reset the proposing_ state of consensus

src/ripple/app/ledger/impl/LedgerMaster.cpp

@@ -828,6 +828,8 @@ LedgerMaster::consensusBuilt(
     if (standalone_)
         return;
 
+    mLedgerHistory.builtLedger (ledger, std::move (consensus));
+
     if (ledger->info().seq <= mValidLedgerSeq)
     {
         auto stream = app_.journal ("LedgerConsensus").info();
@@ -912,8 +914,6 @@ LedgerMaster::consensusBuilt(
             << "Consensus triggered check of ledger";
         checkAccept (maxLedger, maxSeq);
     }
-
-    mLedgerHistory.builtLedger (ledger, std::move (consensus));
 }
 
 void