Add support for @Priority in selection of message body readers and writers.

"Application-supplied providers may be annotated with @Priority . If two or more providers are candidates
for a certain task, the one with the highest priority is chosen: the highest priority is defined to be the one
with the lowest value in this case. That is, @Priority(1) is higher than @Priority(10) . If two or more
providers are eligible and have identical priorities, one is chosen in an implementation dependent manner."