Throttle the reading when writes are asynchronous

Throttle the reading when writes are asynchronous

Be more aggressive when throttling the reading - when we can't send the
outgoing TCP synchronously with uv_try_write(), we start throttling the
reading immediately instead of waiting for the send buffers to fill up.

This should not affect behaved clients that read the data from the TCP
on the other end.

(cherry picked from commit bc3e713317df838b4bbe6582b357cd8d78e585cd)