Add support for getblockchaininfo rpc cmd

-sync.supply setting now allows getting moneysupply from getblockchaininfo cmd
-README updated with info abour getblockchaininfo cmd

lib/settings.js

@@ -1123,6 +1123,7 @@ exports.sync = {
   //         Valid options:
   //         COINBASE : retrieve the total coins sent from the coinbase (Often used for PoW coins)
   //         GETINFO : retrieved from getinfo rpc cmd (Often used for PoS coins)
+  //         GETBLOCKCHAININFO : retrieved from getblockchaininfo rpc cmd
   //         HEAVY: retrieved from getsupply rpc cmd (The "blockchain_specific.heavycoin.enabled" setting must be set to true and the "blockchain_specific.heavycoin.api_cmds.getsupply" setting must be set up correctly for this option to work properly)
   //         BALANCES : get the supply by running a query on the addresses collection and summing up all positive balances (potentially a long running query for blockchains with tons of addresses)
   //         TXOUTSET : retrieved from gettxoutsetinfo rpc cmd
@@ -1160,6 +1161,8 @@ exports.api_cmds = {
   "getrawtransaction": "getrawtransaction",
   // getinfo: Returns an object containing various state info
   "getinfo": "getinfo",
+  // getblockchaininfo: Returns an object containing various state info regarding blockchain processing
+  "getblockchaininfo": "getblockchaininfo",
   // getpeerinfo: Returns data about each connected network node as a json array of objects
   "getpeerinfo": "getpeerinfo",
   // gettxoutsetinfo: Returns an object with statistics about the unspent transaction output set