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/rpc/misc.cpp

@@ -69,7 +69,7 @@ UniValue validateaddress(const JSONRPCRequest& request)
     {
 
 #ifdef ENABLE_WALLET
-        if (!::vpwallets.empty() && IsDeprecatedRPCEnabled("validateaddress")) {
+        if (!GetWallets().empty() && IsDeprecatedRPCEnabled("validateaddress")) {
             ret.pushKVs(getaddressinfo(request));
         }
 #endif