SHAMapStore Online Delete (RIPD-415):

Makes rippled configurable to support deletion of all data in its key-value
store (nodestore) and ledger and transaction SQLite databases based on
validated ledger sequence numbers. All records from a specified ledger
and forward shall remain available in the key-value store and SQLite, and
all data prior to that specific ledger may be deleted.

Additionally, the administrator may require that an RPC command be
executed to enable deletion. This is to align data deletion with local
policy.
Updated freeze tests:
* Always run freeze tests (and  enforcement tests)
* book_offers filtering tests are broken
Template abstract_clock on Clock:
The abstract_clock is now templated on a type meeting the requirements of
the Clock concept. It inherits the nested types of the Clock on which it
is based. This resolves a problem with the original design which broke the
type-safety of time_point from different abstract clocks.
Remove unused chrono::time_point stream conversions
Don't call std::exit on clean exit
Add more unit tests to rpc/impl/TransactionSign (RIPD-480):

By adding a mock it is possible to test the transactionSign
function without interacting with the ledger.  This is the
smallest change I could come up with that allows transactionSign
to be unit tested.

The unit tests are white boxed.  Each test case is a result
of examining the code and identifying behavior associated with
different JSON fields.  That means the tests are not based on
requirements, they are based on observed behavior.
Move the call to cancel_timer to the right place
Fixes to beast::asio::streambuf:
* Fix to_string conversion
* Fix assert on debug invariant checks
* Fix the treatment of the output position when the entire output is committed.
* Add unit test
Use injections from OverlayImpl in PeerImp
Use cluster state in Slot instead of PeerImp
Refactor protocol message parsing:

This replaces the stateful class parser with a stateless free function.
The protocol buffer message is parsed using a ZeroCopyInputStream.

* Invoke method is now a free function.
* Protocol handler doesn't need to derive from an abstract interface
* Only up to one message is processed at a time by the invoker.
* Remove error_code return from the handler's message processing functions.
* Add ZeroCopyInputStream implementation that wraps a BufferSequence.
* Free function parses up to one protocol message and calls the handler.
* Message type and size can be calculated from an iterator
  range or a buffer sequence.
Require the master key when performing certain operations (RIPD-666):
* When disabling the use of the master key; or
* When enabling 'no freeze'.
Claim a fee when a required destination tag is not specified (RIPD-574)
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.
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
When pathfinding, don't output a redundant account node
Fix offer->ACCOUNT->offer
Remove support for deprecated PreviousTxnID field (RIPD-710):

The PreviousTxnID field has been deprecated and should not be used for
transactions that use the field will now be rejected.

The AccountTxnID feature should be used instead by enabling transaction
tracking and specifying a transaction ID at submission. More details
are available at: https://ripple.com/build/transactions/#accounttxnid