ACTIVEMQ6-61 SelectorParser package conflict with openwire tests

The SelectorParser class in amq6 source code conflicts
with the same class in activemq 5 client jar. The activemq 5
client jar is needed to run openwire tests where the activemq5
client interacts with amq6 broker. In tests they are usually
in the same VM, so depending on the classpath order, whichever
class is loaded it will be used by both client and broker.
Unfortunately the method parse() in the class has different
return types in the client jar and broker side jar.
That will cause NoSuchMethod exception.

The fix moves the broker side class into a different package, i.e.
from
org.apache.activemq.selector.SelectorParser
to
org.apache.activemq.selector.impl.SelectorParser