PipeManager stop method segmentation fault

[davidcassany]

testtranscoder produces a segmentation fault when running "pipe->stop()", it is an issue located in LiveMediaFilter's destructor.

[gerardcl]

@davidcassany issue solved but found a new issue related to assure all elements cleanup when stopping the pipe.

Problem statement: At branch development and commit 68189e7 when executing stop event from API and restarting any element inside it crashes. Also, if executing stop event consecutively, second time tcp socket does not return any message, creating a blocking situation.