Reapply trace and span ID formatting optimization in opencensus-contrib-log-correlation-log4j2

[sebright2]

This issue is for reapplying the optimization that was removed in #1437. The optimization moved trace and span ID formatting off of the critical path by inserting lazy formatting objects into the Log4j context, but it caused a ClassCastException in Log4j (#1436). I also opened a Log4j issue: https://issues.apache.org/jira/browse/LOG4J2-2449. When the issue is resolved, we could make opencensus-contrib-log-correlation-log4j2 require a later version of Log4j and reapply the optimization.

/cc @bogdandrutu @g-easy