Lockup in the print spooler.

A recent change modified the way we destroy the remote renderer from
asynchronous to synchronous. This caused problems since it was possible
that the remote rendering service is unbound while we are reading the
contents of a rendered page. As a result the reader was blocking on I/O
and the print spooler was getting into a locked state that required a
restart of its process.  Now the remote renderer is destroyed
asynchronously.

bug:18498626

Change-Id: I1312bf808f30430728b4038dd4be43c55d2be825