Make the broker relay heartbeat intervals configurable

Prior to this commit, the intervals at which the broker relay's system
session would send heartbeats to the STOMP broker and expect to
receive heartbeats from the STOMP broker were hard-coded at 10
seconds.

This commit makes the intervals configurable, with 10 seconds being
the default value.
Introduce new HEARTBEAT message type

Previously, there was no generic concept of a message that represents
a heartbeat and the STOMP-specific code used a null STOMP command to
represent a heartbeat.

This commit introduces HEARTBEAT as a new SimpMessageType. The STOMP
support has been updated to create HEARTBEAT messages to represent
heartbeats, and to use the new message type as the mechanism by which
heartbeats are identified.
Add javax.servlet.http.Part support for data binding

Prior to this commit, Multipart databinding would only support
MultiPartFile databinding using commons-multipart.

Now the WebRequestDataBinder supports Part and List<Part>
databinding for Servlet 3.0 compliant containers.

Issue: SPR-10591
Add Jackson's Modules registration in Jackson2OMFactoryBean

Prior to this commit, one couldn't configure Jackson's ObjectMapper
with (De)SerializerModifiers or advanced configuration features
using XML configuration.

This commit updates the FactoryBean and adds a setModule method
that will register Modules with the ObjectMapper.

Note that this commit is only about XML configuration, since
this feature was already available with JavaConfig.

Issue: SPR-10429