Even before this change SockJS sessions always cancelled the heartbeat
task first prior to sending messages. However when the heartbeat task
is already in progress, cancellation of it is not enough and we must
wait until the heartbeat is sent.

This commit adds a heartbeat write lock which is obtained and held
during the sending of a heartbeat. Now when sessions send a message
they still cancel the heartbeat task but if that fails they also wait
for the heartbeat write lock.

Issue: SPR-14356

Consistent use of BDDMockito rather than standard Mockito.

sockjs-client expects a prelude to be written on every request with
streaming transports. The protocol tests don't make this clear and
don't expose this issue.

The test case for SPR-11183 (writing 20K messages in succession) did
expose the issue and this commit addresses it.

Issue: SPR-11183

Introduced SockJsSession interface and moved SockJsSessionFactory and SockJsServiceConfig to sockjs.transport; added initialize(SockJsServiceConfig) method to TransportHandler interface; extracted TransportHandlingSockJsService from DefaultSockJsService; moved sockjs.support.frame to sockjs.frame and extracted (Default)SockJsFrameFormat from SockJsFrame; moved SockJsHttpRequestHandler to sockjs.support; removed Jackson 1.x support

Consistent ".jetty" and ".standard" subpackages; consolidated GlassFishRequestUpgradeStrategy implementation; renamed Text/BinaryWebSocketHandler and moved them to web.socket.support

After this change the DefaultHandshakeHandler delegates to a
server-specific RequestUpgradeStrategy to update the HTTP response for
the handshake request and to begin the WebSocket interaction.

The DefaultHandshakeHandler however still retains the initial
validation of the WebSocket handshake including negotation of origin,
sub-protocol, etc. This allows sub-classes to override various
aspects of the negotiation independant of the WebSocket engine.

A HandshakeInterceptor can be used to intercept WebSocket handshakes
(or SockJS requests where a new session is created) in order to
inspect the request and response before and after the handshake
including the ability to pass attributes to the WebSocketHandler,
which the hander can access through
WebSocketSession.getHandshakeAttributes()

An HttpSessionHandshakeInterceptor is available that can copy
attributes from the HTTP session to make them available to the
WebSocket session.

Issue: SPR-10624

See javadoc in SockJsService for details.

Also remove ReadOnlyMultiValueMap, CollectionUtils has a method for
that already.

After this change the top-level sockjs package contains the main types
for use in applications.

Minor polish to formatting and assertion messages.