* GH-2873: Preserve mapping order in the router
Fixes https://github.com/spring-projects/spring-integration/issues/2873
Sometime it is important to map to most specific exception instead of
its super class.
* Use `LinkedHashMap` for mapping keys in the
`ErrorMessageExceptionTypeRouter`, as well as in its
`AbstractMappingMessageRouter` superclass.
Since we don't do that internal map modification, there is no reason to
worry about concurrent access: we just replace an internal instance
atomically with a new `LinkedHashMap` every time we modify a mapping
for router
**Cherry-pick to 5.1.x**
* * Fix `RouterSpec.RouterMappingProvider` to `LinkedHashMap` as well
* * Fix `RouterTests` for proper mapping order
* Polishing for `AbstractMappingMessageRouter` hierarchy, so we don't
use `protected channelMappings` field access any more
23a73fab87