INT-3308 SubscribableChannel Optimization

JIRA: https://jira.springsource.org/browse/INT-3308

INT-3308 Move Optimization to AbstractDispatcher

- avoids the need for `canShortCutDispatcher()`
- optimizes all framework channels, including AMQP, JMS
- slight additional performance improvement

INT-3308 FinalSingleHandlerChannel

This is still a work in process.

TODO:Parser, parser tests.

See the DirectChannelTests for perforance results.

INT-3308 'Final' Channel Parser Changes, Tests

INT-3308 Polishing; PR Comments

INT-3308 Polishing

* Rename 'final' attribute to 'fixed-subscriber'
* Rename `BasicSingleFixedSubscriberChannel` to `FixedSubscriberChannel`

INT-3308 Polishing

* Rename test cases to match channel
* Fix parser error messages
* Add more exclusive attribute/element tests

INT-3308 More Polishing

* Remove remaining textual references to 'final'
* WARN log if the single subscriber has auto-startup="false"
* Handle the case when a fixed subscriber channel is declared after its handler
* Support the use of a fixed subscriber channel for elements created by an AbstractOutboundChannelAdapterParser