Fixed LOG4J2-512 (Part 1-Code): Exposed Log4j web support interface and methods and the LoggerContext through ServletContext attributes so that threads not affected by filters (such as asynchronous threads) can utilize the LoggerContext. Also updated the Log4j filter so that it supports async. Will update documentation in Part 2 commit.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1561956 13f79535-47bb-0310-9956-ffa450edef68