This stores transaction templates temporarily until their locktime
is reached, avoiding broadcasting (or RBF bumping) transactions
prior to their locktime. For those broadcasting transactions
(potentially indirectly) via Bitcoin Core RPC, this ensures no
automated rebroadcast of transactions on the client side is
required to get transactions confirmed.
This somewhat cleans up the public API of PackageSolvingData to
make it harder to get an invalid amount and use it, adding further
debug assertion to check it at test-time.