INT-3358: Make `MutableMessage` package protected

JIRA: https://jira.spring.io/browse/INT-3358
INT-3353 Fix Docbook White Space

JIRA: https://jira.spring.io/browse/INT-3355
INT-3355 Improve Redis Tests, RedisLockReg.list()

JIRA: https://jira.spring.io/browse/INT-3355

Previously, the RedisAvailableTests unconditionally
flushed Redis; this is not appropriate and could affect
other builds on the CI server (as well as wiping out the
developer's Redis DB).

Change the tests to clean up their own redis data.

Also, improve `RedisLockRegistry.listLocks()` to use mget.