Revisit JMS support

Since ActiveMQ 5.8.0, the modules structure has been revisited and
activemq-core no longer exists. The activemq-broker is required to
create an embedded broker. Since Boot creates such broker by default
if ConnectionFactory is present, a condition has been added to do so
only when the necessary classes are present in the classpath.

The default embedded broker is now configured to disable message
persistence altogether as this requires an extra jar since 5.8.0, i.e.
activemq-kahadb-store.

Split the ActiveMQ auto configuration from the JmsTemplate auto
configuration so these are totally independent.
ActiveMQAutoConfiguration has been created to detect and configure
the ActiveMQ broker if necessary.

The brokerUrl parameter was ignored as long as the inMemory parameter
was true. The actual brokerUrl to use is now determined by the user
defined values of those parameters: if the brokerUrl is set, it is always
used. If no brokerUrl is set, the value of inMemory determines if an
embedded broker should be used (true) or a tcp connection to an
existing local broker (false).

JmsTemplateAutoConfiguration now creates a JmsTemplate only if a
ConnectionFactory is available.

Fixes gh-872, gh-882, gh-883

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -200,7 +200,7 @@ content into your application; rather pick only the properties that you need.
 	spring.activemq.broker-url=tcp://localhost:61616 # connection URL
     spring.activemq.user=
     spring.activemq.password=
-	spring.activemq.in-memory=true
+	spring.activemq.in-memory=true # broker kind to create if no broker-url is specified
 	spring.activemq.pooled=false
 
 	# JMS ({sc-spring-boot-autoconfigure}/jms/JmsTemplateProperties.{sc-ext}[JmsTemplateProperties])