Now all accesses to quark_notify_queue are guarded by the GData lock.
Several non-trivial operations are implemented via
g_datalist_id_update_atomic().
The OPTIONAL_BIT_LOCK_NOTIFY lock is thus unnecessary and can be dropped.
Note that with the move to g_datalist_id_update_atomic(), we now
potentially do more work while holding the GData lock (e.g. some code
paths allocation additional memory). But note that
g_datalist_id_set_data() already has code paths where it must allocate
memory to track the GDataElt. Also, most objects are not used in
parallel, so holding the per-object (per-GData) lock longer does not
affect them. Also, many operations also require a object_bit_lock(), so
it's unlikely that you really could achieve higher parallelism by taking
more locks (and minimizing the time to hold the GData lock).
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