修复ConcurrentModificationException的异常，当请求量比较大的时候
Update WebAppStat.java

去除多余的注释
fix #535
Merge pull request #543 from bucketli/master

fix #535
Merge pull request #512 from hadeslee/master

当session很多的时候，修复ConcurrentModificationException异常
Merge remote-tracking branch 'upstream/master'