Use importmulti timestamp when importing watch only keys

When importing a watch-only address over importmulti with a specific timestamp,
the wallet's nTimeFirstKey is currently set to 1. After this change, the
provided timestamp will be used and stored as metadata associated with
watch-only key. This can improve wallet performance because it can avoid the
need to scan the entire blockchain for watch only addresses when timestamps are
provided.

Also adds timestamp to validateaddress return value (needed for tests).

Fixes #9034.

src/rpc/misc.cpp

@@ -208,6 +208,9 @@ UniValue validateaddress(const JSONRPCRequest& request)
         if (pwalletMain) {
             const auto& meta = pwalletMain->mapKeyMetadata;
             auto it = address.GetKeyID(keyID) ? meta.find(keyID) : meta.end();
+            if (it == meta.end()) {
+                it = meta.find(CScriptID(scriptPubKey));
+            }
             if (it != meta.end()) {
                 ret.push_back(Pair("timestamp", it->second.nCreateTime));
                 if (!it->second.hdKeypath.empty()) {