Fix bug in transaction_builder

Sometimes the builder generated a bad transaction that overwithdrew from
a balance. This is now fixed.
Update cancel_market_orders to use transaction_builder
Add wallet::create_transaction_builder

Simple utility function to create and return a transaction_builder.
Important if external code ever wants to use one, since the builder
needs a pointer to the wallet_impl, which is not available to external
code.
Merge branch 'develop' into short-interest
Enforce APR limit