Keepalive not sent when updates are being processed

To hit this bug, configure a BGP speaker to have HoldTimer set to 3 seconds and then point it to ODL. Then let the BGP speaker wait about 0.5 seconds and then send 1750 updates. If you have topology updates turned OFF in the ODL instance, you can ask the BGP speaker to send up to 5000 updates.

The speaker won't see any KeepAlive messages coming from ODL while it is sending the updates, thus closing the connection down.

The problem can happen with other HoldTimer values as well. If the speaker starts sending a lot of updates at the time when the HoldTimer is nearly expired, then ODL is going to miss the HoldTimer deadline.

This may be related to YANGTOOLS-383 but I am not sure.