Fix issue in htmlfile sockjs transport

Issue: SPR-10854
Improve handling of send failures

Prior to this commit, a failure to send a heartbeat was ignored and a
failure to forward a message to the broker would result in an error
frame being sent but nothing more.

Following this commit, a failure to send a heartbeat to the broker
is treated as a TCP client failure. Furthermore, if the system relay
session fails to forward a message to the broker an exception is
thrown. Typically, the system relay session will be forwarding
messages on behalf of local application code, rather than a remote
WebSocket client. Throwing an exception allows the application code
to be notified of the problem directly, rather than via a broker
availability event.
Introduce CONNECT_ACK message type

Previously, handling of a STOMP CONNECT message and sending of a
CONNECTED response was performed by StompProtocolHandler if it was
backed by SimpleBrokerMessageHandler, or left up to the real message
broker if it was backed by StompBrokerRelayMessageHandler. This
wasn't ideal as it should be StompProtocolHandler's job to simply map
messages to and from the STOMP protocol, not to do part of the
broker's job and respond directly to CONNECT.

This commit introduces a new message type, CONNECT_ACK. When it
receives a CONNECT message, SimpleBrokerMessageHandler will now
respond with a CONNECT_ACK message that StompProtocolHandler can map
into a STOMP CONNECTED message. The CONNECT_ACK message contains the
CONNECT message as a header so that StompProtocolHandler has access to
its accept-version header.

StompProtocolHandler has been simplified so that a CONNECT message
is always passed to the output channel, irrespective of whether it's
backed by a simple broker or a real broker. The handleConnect flag,
and the code that would set it correctly depending on the app's
configuration, has been removed.
Polishing

 - Polish javadoc for CONNECTED_USER_HEADER
 - Improve method ordering
Polish and fix issues in STOMP broker relay

Fix error in te code that handles the result of sending a heartbeat

Fix error in processing DISCONNECTED frames that closed the TCP
connection before the message was sent.
Fix issue in simple broker with peer-to-peer messages

Issue: SPR-10930
Fix issue with getting SockJS path

Issue: SPR-10923
Prevent cache of sockjs known prefixes from growing

Issue: SPR-10893