bbr: fix cwnd calculation when recovery starts

[junhochoi]

Alternative to #1473. Also fixed the test which I believe it's correct.

Previously bbr_enter_recovery() used old newly_acked_bytes to set a new cwnd when the recovery starts. To prevent this from happening, bbr_enter_recovery() is called after newly_acked_bytes is updated in a current ack processing. Also bbr_modulate_cwnd_for_recovery() use the same cwnd calculation to match the same in bbr_enter_recovery().

@lohith-bellad please feel free to take a look if it makes sense. I didn't test it much