For MEETs, save the extensions support flag immediately during MEET processing

[bentotten]

For backwards compatibility reasons, a node will wait until it receives a cluster message with the extensions flag before sending its own extensions. This leads to a delay in shard ID propagation that can corrupt nodes.conf with inaccurate shard IDs if a node is restarted before this can stabilize.

This fixes much of that delay by immediately triggering the extensions-supported flag during the MEET processing and attaching the node to the link, allowing the PONG reply to contain OSS extensions.

Partially fixes #774