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.
Additionally, accept a "now" timestamp, to allow avoiding rescans for keys
which are known never to have been used.
Note that the behavior when "now" is specified is slightly different than the
previous behavior when no timestamp was specified at all. Previously, when no
timestamp was specified, it would avoid rescanning during the importmulti call,
but set the key's nCreateTime value to 1, which would not prevent future block
reads in later ScanForWalletTransactions calls. With this change, passing a
"now" timestamp will set the key's nCreateTime to the current block time
instead of 1.
Fixes#9491
