Synchronize access to the underlying object of Slice

Synchronize access to the underlying object of Slice

This was prompted by the interactions between GL::ResourceManager and
some upcoming changes to DataFile::PackSource.  If a new Slice object is
created and tries to connect to signal_flush_required at the exact moment
another Slice is seeking and causing the signal to be emitted, the emit
and connect calls step on each others' toes and corrupt the heap.