wallet: Add AddWallet, RemoveWallet, GetWallet and GetWallets

With these new functions all vpwallets usage are removed
and vpwallets is now a static variable (no external linkage).

src/interfaces/node.cpp

@@ -239,7 +239,7 @@ class NodeImpl : public Node
     {
 #ifdef ENABLE_WALLET
         std::vector<std::unique_ptr<Wallet>> wallets;
-        for (CWallet* wallet : ::vpwallets) {
+        for (CWallet* wallet : GetWallets()) {
             wallets.emplace_back(MakeWallet(*wallet));
         }
         return wallets;