Signers: Share keypaths for watching wallets

Now LocalTransactionSigner populates ProposedTransaction.keyPaths map even if
there is no private key in the wallet. Other signers may take advantage
of that.
Postgres store: Simplify a redundant if conditional.
Signers: Share keypaths for watching wallets

Now LocalTransactionSigner populates ProposedTransaction.keyPaths map even if
there is no private key in the wallet. Other signers may take advantage
of that.