Revert "Remove DocumentHandler.AsyncPusher interface"

This reverts commit 4adc1004c67d2be21639ad7a7473ff69f7677946.

This CL exoses a failure where Item was visible outside of
library package.  Further thinking about this CL revelaed
that DocumentHandler.AsyncPusher had a purpose of hiding
the pushing of fragments (which are pushed because of GSA
limitations).