Unify properties into a LoggerContext-scoped properties system

[jvz]

https://cwiki.apache.org/confluence/display/LOGGING/Properties+Enhancement

Also, first!

[ppkarwasz]

@jvz, didn't we complete this one?

[jvz]

Generally speaking, yes, though I was still going to investigate some straggler properties that don't actually support per-context properties but should.

[jvz]

Actually, this was completed in https://issues.apache.org/jira/browse/LOG4J2-3299 and in #1473.