This changes the wtxmgr.Store.UnminedTxs method to sort transactions,
using the Kahn topological sort algorithm, before returning
transactions to the caller. This is possible because transactions
form a sort of directed acyclic graph (DAG) where transactions
reference other transactions to spend their outputs (multiple
referenced outputs from a single transaction spent by the same
transaction count as a single graph edge).
This prevents the possibility of orphan rejection errors when sending
unmined transactions to a full node at startup. As these transactions
are sent using the sendrawtransaction RPC, which does not permit
orphans, this topological sort is required.
Fixes#156.
