Replace on_end_headers_callback with on_begin_headers_callback

Previously, there is inconsistency when on_frame_recv_callback
is called between HEADERS/PUSH_PROMISE and the other frames.
For former case, it is called before header block, in latter
case, it is called after whole frame is received. To make it
consistent, we call on_frame_recv_callback for HEADERS/PUSH_PROMISE
after its frame is fully received. Since on_frame_recv_callback
can signal the end of header block, we replaced on_end_headers_callback
with on_begin_headers_callback, which is called when the reception
of the header block is started.
Call on_frame_recv_callback for DATA even if stream has been closed

This makes the callback semantics consistent with the other
frames like HEADERS/PUSH_PROMISE.
Update doc