Fix missing 'else' when handling sfMessageKey:
When clearing out a message key the transactor would incorrectly
create an empty `sfMessageKey` field instead of simply deleting
the field.

[Transaction Processing Change]
Clarify logic and clean up existing code
Add simplified explicit interfaces to handle XRP and IOU transfers:

The new interfaces take into account the different semantics of XRP, which
do not have an issuer or transfer fees, and IOUs which have issuers and
(optional) transfer fees.

For XRP, the new `LedgerEntrySet::transfer_xrp` will transfer the specified
amount of XRP between from a given source to a given destination.

For IOU, two new functions are introduced:
* `LedgerEntrySet::issue_iou` which transfers the specified amount of an
IOU from the IOU's issuer to an account.
* `LedgerEntrySet::redeem_iou` which transfers the specified amount of an
IOU from an account to the IOU's issuer.

A transfer from user-to-user (e.g. to fill an order during offer crossing)
requires the use of `redeem_iou` followed by `issue_iou`. This helps to
enforce the Ripple invariant that IOUs never flow directly from user to
user, but only through a gateway. Additionally, this  allows for the
explicit calculation and application of transfer fees by varying the
amounts redeemed and issued.

The new interfaces promote type safety since you cannot use the issue
and redeem APIs with XRP or the transfer API with IOU, and the issuer
to be used is implied by the currency being issued or redeemed.
Finalize autobridging implementation (RIPD-179):

Autobridging uses XRP as a natural bridge currency to allow IOU-to-IOU orders
to be satisfied not only from the direct IOU-to-IOU books but also over the
combined IOU-to-XRP and XRP-to-IOU books.

This commit addresses the following issues:

* 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