Finalize autobridging implementation (RIPD-179):

* RIPD-486: Refactoring the taker into a unit-testable architecture
* RIPD-659: Asset-aware offer crossing
* RIPD-491: Unit tests for IOU to XRP, XRP to IOU and IOU to IOU
* RIPD-441: Handle case when autobridging over same owner offers
* RIPD-665: Handle case when autobridging over own offers
* RIPD-273: Groom order books while crossing

Additionally, add support in LedgerEntrySet to allow transfer fees
to be specified by calling code instead of being calculated by the
transfer functions:

* Implement new "issue/redeem" model for IOU movements
* Implement new "transfer" model for IOU movements

Clean up signature verification logic in the main Transactor.

Reviews: @JoelKatz, @rec, @vinniefalco but everyone is welcome to get in on the game.
There's really nothing to tell. This is a fatal error that should **never**, under any circumstances, occur. Our callers never call us in this way, and if they do it signifies an internal error and we don't know what to do other than give up and refuse to process.