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/wallet/walletdb.cpp

@@ -756,7 +756,7 @@ void MaybeCompactWalletDB()
         return;
     }
 
-    for (CWallet* pwallet : vpwallets) {
+    for (CWallet* pwallet : GetWallets()) {
         WalletDatabase& dbh = pwallet->GetDBHandle();
 
         unsigned int nUpdateCounter = dbh.nUpdateCounter;