Packets still lost due to buffer overflow at connection to new Region

[AndrewMeadows]

Environment

All official SL viewers, all servers.

Description

When the Viewer connects to a new Region (either by login/teleport/vew-into-neighbor-rgion) the simulator running the Region will usually send a burst of UDP messages to the Viewer -- so many that it exceed the Viewer's ability to process them and the spike of network traffic can overflow the inbound buffer. In PR #3565 there was an attempt to reduce packet loss in this case, however those measures were insufficient: we still lose packets , which continue to be a source (not the only one) of non-visible content and cloudy avatars.

We should increase the size of the buffer and try very hard to NEVER drop packets. The moment packets are queued they should be ACKed (which will reduce resends) and harvested for outbound ACKs (which will prevent unnecessary outbound resends).

Reproduction steps

1. Login to a very busy region (lots of avatar) with lots of content.
2. Notice packet loss in the stats.
3. Look in logs for messages about lost packets.

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