Revert "gnlobject: Commit object in READY_TO_PAUSED"

This causes races when seeking, reverting for now even if we will
probably want to bring something like that back.

This reverts commit 3549e745a8f0de3977b83c60e9b447afaf55d8a0.
gnl: Add the srcpad directly to GnlObject

Starting from now we will not claim that we support GnlObject that have
several source pads as this is
1- Not true at all;
2- the design of priorities in the GnlComposition tree does not allow that;
3- Not very useful in most of the cases and it complexifies quite a lot the code
   in the composition.
gnlcomposition: Add a function to block object source pad
gnlcomposition: Factor out the condition of _parent_or_priority_changed
gnlcomposition: factor out some functions
gnlcomposition: Use the new _object_block_and_drop_data where appropriate

Renaming block_object_src_pad to _object_block_and_drop_data
composition: Avoid looping using gotos
gnlcomposition: Factor out code to deactivate old stack and activate new one
composition: Use a GstPad task to run the update pipeline thread
composition: Reset the srcpad target when removing the toplevelentry
composition: simplify update_pipeline_func
WIP for commit func
Don't be sync silly test
Revert "WIP for commit func"

This reverts commit 384c69862d4d74c0df55ddca2b55ec4d62d80963.
composition: home grown task
tests: unref message correctly
composition: start and stop task in init and finalize