fix segfault and fd leaking problems

[zhao85]

1. to avoid bus fd leak, removed DUP fd reference and added bus watch removing in dispose of class
2. switched the release position of appsrc and appsink in pad removing processing

[jenkinskurento]

Hi there, thanks for your Pull Request!

A Kurento member needs to verify that this patch is reasonable to test. In case it is, they should write a comment with the phrase test this please. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by Kurento members will still work. Regular contributors can be whitelisted to skip this step.