Fix the memory leak of jvm and jwsgi plugins

[mountain]

Hi, folks,

This is my fix for the memory leak of jvm and jwsgi plugins by using the ways:

• all jobjects returned by jvm should be a local ref
• use push/pop to manage local frame
• release string after using them

I had tested this code on Ubuntu 12.04 and Oracle JDK 7 with uwsgi head.

I use ab under 100 concurrent level to check the memory leak, after half an hour, the memory usage are stable.

Hope this fix is helpful.