Instead of looking at the cached amounts or searching every input of a
transaction each time we want to determine whether it is "from me", use
m_from_me  which stores this value for us.

Perform the transaction reorder upgrade immediately after loading txs
instead of waiting for the end of loading.

Since we need to know whether the transaction that creates a WalletTXO
is "from me", we should store this state in the WalletTXO too, copied
from its parent CWalletTx.

WalletTXOs need to know their parent tx's timestamp for AvailableCoins
to work.

When the state of a CWalletTx changes, we need to change the state in
the WalletTXOs too.

A min_conf parameter is added to IsSpent so that it can set a
confirmation threshold for whether something is considered spent.

When a block is disconnected, we need to process the transactions in
reverse order so that the wallet's TXO set is updated in the correct
order.

CWallet::Create will properly connect the wallet to the chain, so we
should be doing that rather than ad-hoc chain connection.

Definitely unusable TXOs are those that are spent by a confirmed
transaction or were produced by a now conflicted transaction. However,
we still need them for GetDebit, so we store them in a separate
m_unusable_txos container. MarkConflicted, AbandonTransaction, and
loading (via PruneSpentTXOs) will ensure that these unusable TXOs are
properly moved.