Merge #7648: BIP9 versionbits softfork for BIP68, BIP112 and BIP113

71527a0 Test of BIP9 fork activation of mtp, csv, sequence_lock (NicolasDorier)
19d73d5 Add RPC test for BIP 68/112/113 soft fork. (Alex Morcos)
12c89c9 Policy: allow transaction version 2 relay policy. (BtcDrak)
02c2435 Soft fork logic for BIP68 (BtcDrak)
478fba6 Soft fork logic for BIP113 (BtcDrak)
65751a3 Add CHECKSEQUENCEVERIFY softfork through BIP9 (Pieter Wuille)

src/primitives/transaction.h

@@ -207,8 +207,15 @@ private:
     void UpdateHash() const;
 
 public:
+    // Default transaction version.
     static const int32_t CURRENT_VERSION=1;
 
+    // Changing the default transaction version requires a two step process: first
+    // adapting relay policy by bumping MAX_STANDARD_VERSION, and then later date
+    // bumping the default CURRENT_VERSION at which point both CURRENT_VERSION and
+    // MAX_STANDARD_VERSION will be equal.
+    static const int32_t MAX_STANDARD_VERSION=2;
+
     // The local variables are made const to prevent unintended modification
     // without updating the cached hash value. However, CTransaction is not
     // actually immutable; deserialization and assignment are implemented,