Better support for HornetQ embedded broker

Prior to this commit it was not safe to start several contexts
using the HornetQAutoConfiguration in the same VM. Each context
was trying to start their own HornetQ embedded broker by default but
only the first was really starting. Worse, the various InVM connection
factories were all silently connecting to the first broker.

This commit introduces a new "serverId" property that is an auto-
incremented integer by default. This identifies the server to connect
to and allows each context to start its own embedded broker in total
isolation of other contexts.

This commits makes it possible for a context to disable its own
embedded broker and connect to an existing one, potentially started
by another context.

Fixes gh-1063