Defensive coding around some XML activities that are triggered by web applications and are therefore at potential risk of a memory leak.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1590028 13f79535-47bb-0310-9956-ffa450edef68
Ensure TLD parser obtained from cache has correct value of blockExternal

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1590036 13f79535-47bb-0310-9956-ffa450edef68
Followup to r1590036 
1) simplify
2) add 'synchronized', as web applications may start in parallel

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1590040 13f79535-47bb-0310-9956-ffa450edef68