composition: Avoid deadlock when setting current bin to ready (on commit or seek)

We need to make sure that between the time we send flush_start/stop and
the time we actually set the bin to READY, no buffer got prerolled again
as it would lead to a deadlock trying to set the bin to READY (while
deactivating the pads, it needs the streaming lock, which would be
taken in that case)
source: Atomically change the probe ID

Avoiding races where the probe would be removed 2 times
composition: Push flush events *downstream* not 'somewhere' :)
composition: Add the stack start/stop that has been set in dotfile name
composition: Do not use 'update' seek for now

This is not working in our new context and the seek do not work at
all when we set seek start to CLOCK_TIME_NONE and type to TYPE_NONE.
tests: Deactivate seeking tests, their are racy and not clean.
composition: remove now useless "flushing" flag.