QUIC: stream flow control refactored.

QUIC: stream flow control refactored.

- Function ngx_quic_control_flow() is introduced.  This functions does
both MAX_DATA and MAX_STREAM_DATA flow controls.  The function is called
from STREAM and RESET_STREAM frame handlers.  Previously, flow control
was only accounted for STREAM.  Also, MAX_DATA flow control was not accounted
at all.

- Function ngx_quic_update_flow() is introduced.  This function advances flow
control windows and sends MAX_DATA/MAX_STREAM_DATA.  The function is called
from RESET_STREAM frame handler, stream cleanup handler and stream recv()
handler.