[Wallet] sort pending wallet transactions before reaccepting

During startup, when adding pending wallet transactions, which spend outputs of
other pending wallet transactions, back to the memory pool, and when they are
added out of order, it appears as if they are orphans with missing inputs.

Those transactions are then rejected and flagged as "conflicting" (= not in the
memory pool, not in the block chain).

To prevent this, transactions are explicitly sorted.