INT-2371 DefaultHeaderChannelRegistry Improvements

JIRA: https://jira.spring.io/browse/INT-3471

- Make final fields protected so they are available to subclasses
- Add property `removeOnGet` allowing the map entry to be removed immediately when it is used
- Add `time-to-live-expression` allowing override of the reaper delay

Polishing

Conflicts:
	src/reference/docbook/whats-new.xml