Add is_short_read()
Fix handling of HTTP/S keep-alives (RIPD-556):
* Proper shutdown for ssl and non-ssl connections
* Report session id in history
* Report histogram of requests per session
* Change print name to 'http'
* Split logging into "HTTP" and "HTTP-RPC" partitions
* More logging and refinement of logging severities
* Log the request count when a session is destroyed

Conflicts:
	src/ripple/http/impl/Peer.cpp
	src/ripple/http/impl/Peer.h
	src/ripple/http/impl/ServerImpl.cpp
	src/ripple/module/app/main/Application.cpp
	src/ripple/module/app/main/RPCHTTPServer.cpp
Set version to 0.26.3-sp1
Merge branch 'release-next' into develop-next