Tighten connection management in STOMP broker relay

Fix NPE exception when closing TcpConnection

Ensure a ConnectionHandler is cleared when a TcpConnection is closed
(at the same time), logging an exception if the closing fails.

Improve error messages.

Issue: SPR-11531
Add messages for number of active sessions/connections

Issue: SPR-11531
Handle invalid STOMP content-length header value

After this change if a content-length header is provided but is less
than 0 or cannot be parsed as a number, it is ignored and the body
is read sequentially, i.e. until we reach a null byte terminator.

This provides better protection against clients that may set the
content-length header in error.

Issue: SPR-11528
SpEL property-not-found message hints at non-public method/field

Issue: SPR-11535
(cherry picked from commit eeb82c4)
Upgraded to Apache HttpClient 4.3.3 and Jetty 9.1.3
Fix "relay-port" XSD type in spring-websocket.xsd
Prior to this commit, the `relay-port` attribute of the
`<websocket:stomp-broker-relay />` tag was of type `xsd:int`.
This prevents developers from using `PropertyPlaceholderConfigurer`,
even though this configuration key is a good candidate for such use
(this value depends on prod/staging/etc environment).

This commit changes that type to `xsd:string`.

Issue: SPR-11537
Polishing