Multi-level deprecation management

[Sylvain Wallez <sy...@apache.org>]

Hi all,

I refactored the DeprecationLogger class, which is now in 
o.a.c.util.Deprecation.

Rather than a single log() method, all levels of the static 
Deprecation.logger can be used to indicate various levels of deprecation.

The javadocs give some hints on how to use each level:
- debug: no more current or best practice, but no plans to remove the 
feature
- info: no more current or best practice, may be removed in the future
- warn: will be removed in the next major release (e.g. 2.1.x --> 2.2.0)
- error: will be removed in the next minor release (e.g. 2.1.6 --> 2.1.7)
- fatal error: has been removed, but some code is still there to detect 
attempts to use the feature.

In addition to this, a "forbidden deprecation level" has been added, 
leading the logging at this level and above to throw a 
DeprecationException. It can be set in web.xml and defauts to ERROR 
(meaning up to WARN is allowed).

Setting this forbidden level to DEBUG runs the application in strict 
mode (even non-current features are forbidden), and setting it to FATAL 
runs in tolerant mode, i.e. allow everything except, of course, features 
that have been removed.

This should allow us to give a clear indication to our users of what 
they should give attention to if they want their apps to be maintainable 
for a long time.

Sylvain

-- 
Sylvain Wallez                                  Anyware Technologies
http://www.apache.org/~sylvain           http://www.anyware-tech.com
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