Improve the human-readable description of the tesSUCCESS code:

Transactions that return tesSUCCESS have only been accepted and
propagated on the Ripple network and should not be considered
final until they have been included in a validated ledger.
Use asio signal handling in Application (RIPD-140):
* Use signal_set as cross platform way of handling SIGINT
* Remove polling on main thread for shutdown.
* Add extra logging for received signal.
* Clean up exit handling on error in setup routines.
* Reuse isStopped() from Stoppable for status (could be isStopping() instead).
* Ctrl-C should now work for standalone mode as well on Windows.

Also small fixes to Resolver:
* Add Resolver prefix to logging.
* Fix AsyncObject::removeReference() logic.
* Fix work remaining logic.
Add http::chunk_encode:
This transforms a ConstBufferSequence into a new ConstBufferSequence whose
data is encoded according to the Content transfer encoding rules of RFC2616.
The implementation does not copy any memory.
Fix case where slot==nullptr in Overlay:
This changes the Overlay to correctly handle the case when nullptr is
returned by PeerFinder new_inbound_slot on a detected self-connection.
Cleanup ripple::Ledger:
* Convert static member functions to free functions
* Adopt consistent naming convention
* De-inline code
Tidy up the structure of sources in protocol/:
Split out and rename STValidation
Split out and rename STBlob
Split out and rename STAccount
Split out STPathSet
Split STVector256 and move UintTypes to protocol/
Rename to STBase
Rename to STLedgerEntry
Rename to SOTemplate
Rename to STTx
Remove obsolete AgedHistory
Remove types.h and add missing includes
Remove unnecessary includes in app.h
Remove unnecessary includes in app.h
Remove include app.h from app1.cpp
Simplify the Beast fatal error reporting framework:
* Reduce interface to a single function which reports error details
* Remove unused functions
Fix DecayingSample treatment of the window
Fix RPC handlers to use the results of lookupLedger.
Clarify SetAccount logic and clean up existing code
Fix unit_test suite matching with full names
Replace custom exceptions with std::runtime_error
Add WrappedSink:
This class puts a configured string prefix in front of
each line of Journal output.
HTTP Handshaking for Peers on Universal Port (RIPD-446):

This introduces a considerable change in the way that peers handshake. Instead
of sending the TMHello protocol message, the peer making the connection (client
role) sends an HTTP Upgrade request along with some special headers. The peer
acting in the server role sends an HTTP response completing the upgrade and
transition to RTXP (Ripple Transaction Protocol, a.k.a. peer protocol). If the
server has no available slots, then it sends a 503 Service Unavailable HTTP
response with a JSON content-body containing IP addresses of other servers to
try. The information that was previously contained in the TMHello message is
now communicated in the HTTP request and HTTP response including the secure
cookie to prevent man in the middle attacks. This information is documented
in the overlay README.md file.

To prevent disruption on the network, the handshake feature is rolled out in
two parts. This is part 1, where new servents acting in the client role will
send the old style TMHello handshake, and new servents acting in the server
role can automatically detect and accept both the old style TMHello handshake,
or the HTTP request accordingly. This detection happens in the Server module,
which supports the universal port. An experimental .cfg setting allows clients
to instead send HTTP handshakes when establishing peer connections. When this
code has reached a significant fraction of the network, these clients will be
able to establish a connection to the Ripple network using HTTP handshakes.

These changes clean up the handling of the socket for peers. It fixes a long
standing bug in the graceful close sequence, where remaining data such as the
IP addresses of other servers to try, did not get sent. Redundant state
variables for the peer are removed and the treatment of completion handlers is
streamlined. The treatment of SSL short reads and secure shutdown is also fixed.

Logging for the peers in the overlay module are divided into two partitions:
"Peer" and "Protocol". The Peer partition records activity taking place on the
socket while the Protocol partition informs about RTXP specific actions such as
transaction relay, fetch packs, and consensus rounds. The severity on the log
partitions may be adjusted independently to diagnose problems. Every log
message for peers is prefixed with a small, unique integer id in brackets,
to accurately associate log messages with peers.

HTTP handshaking is the first step in implementing the Hub and Spoke feature,
which transforms the network from a homogeneous network where all peers are
the same, into a structured network where peers with above average capabilities
in their ability to process ledgers and transactions self-assemble to form a
backbone of high powered machines which in turn serve a much larger number of
'leaves' with lower capacities with a goal to improve the number of
transactions that may be retired over time.
Cleanup some stray formatting left in logs
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