Add mediantime field to getblockchaininfo RPC call

Useful now that BIP113 is enforced for transactions entering the
mempool. Was previously only (indirectly) available by calling
getblocktemplate, a relatively expensive RPC call.

Author: petertodd

src/rpcblockchain.cpp

@@ -639,6 +639,7 @@ UniValue getblockchaininfo(const UniValue& params, bool fHelp)
     obj.push_back(Pair("headers",               pindexBestHeader ? pindexBestHeader->nHeight : -1));
     obj.push_back(Pair("bestblockhash",         chainActive.Tip()->GetBlockHash().GetHex()));
     obj.push_back(Pair("difficulty",            (double)GetDifficulty()));
+    obj.push_back(Pair("mediantime",            (int64_t)chainActive.Tip()->GetMedianTimePast()));
     obj.push_back(Pair("verificationprogress",  Checkpoints::GuessVerificationProgress(Params().Checkpoints(), chainActive.Tip())));
     obj.push_back(Pair("chainwork",             chainActive.Tip()->nChainWork.GetHex()));
     obj.push_back(Pair("pruned",                fPruneMode));