Improve transaction security
* Check signatures of every transaction on every validator
* Remove obsolete code
* Check transaction status in submit/sign RPC handler
Set version to 0.26.3-sp2
Use trusted validators median fee
Set version to 0.26.3-sp4
Fix incorrect socket closure in Overlay peers:
On Application exit, Overlay was calling PeerImp::close for each peer.
The implementation of PeerImp::close only canceled all pending I/O and did not
call functions necessary for proper transition of Peer state during socket
closure. The correct transition is ensured by calling PeerImp::detach. This
changes PeerImp::close to call PeerImp::detach instead, ensuring that Overlay
invariants are maintained. Specifically, that reference counts for pending I/O
on peers will be correctly unwound by canceling operations and that the Peer
object will be destroyed, thus allowing the Overlay to stop correctly.
Merge branch 'release' into develop
Set version to 0.26.4-rc3
Remove obsolete classes, disable unused code, and tidy up:
* Removed MultiSocket. Code that previously used the MultiSocket now uses
  a combination of boost::asio coroutines and CRTP.
* Sitefiles headers rolled up and directory flattened.
* Disabled Sitefiles use of deprecated HTTPClient.
* Validators headers tidied up.
* Disabled Validators use of deprecated HTTPClient.
Remove unused and obsolete classes and tidy up:
Many classes required to support type-erasure of handlers and boost::asio
types are now obsolete, so these classes and files are removed:
HTTPClientType, FixedInputBuffer, PeerRole, socket_wrapper,
client_session, basic_url, abstract_socket, buffer_sequence, memory_buffer,
enable_wait_for_async, shared_handler, wrap_handler, streambuf,
ContentBodyBuffer, SSLContext, completion-handler based handshake detectors.
These structural changes are made:
* Some missing includes added to headers
* asio module directory flattened