Fix custom config with older HttpClient

Since HttpClient 4.3, custom configuration such as the connection
timeout and the socket timeout are set in a RequestConfig object
stored in the HttpContext.

Unfortunately, older HttpClients are not supporting this
infrastructure and new clients throw an exception when the
deprecated API is used.

This commit detects if the client is an "old" implementation and
set the configuration through the deprecated means to restore
full backward compatibility with these features.

Issue: SPR-11442
Improved documentation on how to convert collections with ConversionService

Issue: SPR-11432
Polishing

Issue: SPR-11442
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