Improve handling of BIP9Deployment limits

Small tweaks by Pieter Wuille.

src/chainparams.cpp

@@ -283,13 +283,13 @@ public:
         consensus.nMinerConfirmationWindow = 144; // Faster than normal for regtest (144 instead of 2016)
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].bit = 28;
         consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nStartTime = 0;
-        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = 999999999999ULL;
+        consensus.vDeployments[Consensus::DEPLOYMENT_TESTDUMMY].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
         consensus.vDeployments[Consensus::DEPLOYMENT_CSV].bit = 0;
         consensus.vDeployments[Consensus::DEPLOYMENT_CSV].nStartTime = 0;
-        consensus.vDeployments[Consensus::DEPLOYMENT_CSV].nTimeout = 999999999999ULL;
+        consensus.vDeployments[Consensus::DEPLOYMENT_CSV].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
         consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].bit = 1;
         consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nStartTime = 0;
-        consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nTimeout = 999999999999ULL;
+        consensus.vDeployments[Consensus::DEPLOYMENT_SEGWIT].nTimeout = Consensus::BIP9Deployment::NO_TIMEOUT;
 
         // The best chain should have at least this much work.
         consensus.nMinimumChainWork = uint256S("0x00");

src/consensus/params.h

@@ -7,6 +7,7 @@
 #define BITCOIN_CONSENSUS_PARAMS_H
 
 #include "uint256.h"
+#include <limits>
 #include <map>
 #include <string>
 
@@ -31,6 +32,9 @@ struct BIP9Deployment {
     int64_t nStartTime;
     /** Timeout/expiry MedianTime for the deployment attempt. */
     int64_t nTimeout;
+
+    /** Constant for nTimeout very far in the future. */
+    static constexpr int64_t NO_TIMEOUT = std::numeric_limits<int64_t>::max();
 };
 
 /**