Wallet: Replace pwalletMain with a vector of wallet pointers

src/wallet/rpcwallet.cpp

@@ -33,7 +33,8 @@
 
 CWallet *GetWalletForJSONRPCRequest(const JSONRPCRequest& request)
 {
-    return pwalletMain;
+    // TODO: Some way to access secondary wallets
+    return vpwallets.empty() ? nullptr : vpwallets[0];
 }
 
 std::string HelpRequiringPassphrase(CWallet * const pwallet)