Request body: reading body buffering in filters.

Request body: reading body buffering in filters.

If a filter wants to buffer the request body during reading (for
example, to check an external scanner), it can now do so.  To make
it possible, the code now checks rb->last_saved (introduced in the
previous change) along with rb->rest == 0.

Since in HTTP/2 this requires flow control to avoid overflowing the
request body buffer, so filters which need buffering have to set
the rb->filter_need_buffering flag on the first filter call.  (Note
that each filter is expected to call the next filter, so all filters
will be able set the flag if needed.)