Fix P-chain Shutdown deadlock

[StephenButtolph]

Why this should be merged

Fixes deadlock observed here.

Currently it's possible that:

1. Shutdown is called with the context lock held
2. Gossip is called
3. Inside Gossip the validator set needs to be recalculated
4. Recalculating the validator set blocks until it can grab the context lock
5. Shutdown blocks until Gossip returns

How this works

The options here are:

1. Release the lock in Shutdown prior to waiting and then re-grabbing it
2. Not blocking on Gossip in Shutdown.

This PR takes approach 2. Technically this could cause unexpected warnings to be logged... I felt like this was better than releasing the context lock in Shutdown. Hopefully all of this will be cleaned up once we push the context lock into the VMs.

How this was tested

• CI