Add non-validator communication

[samliok]

The non-validator communication should be different than the validator one, mainly because non-validators run over multiple epochs and validator sets can change in the meantime. This makes the validators function call return query the pchain.

Also rather than changing the communication api, I made it so errors fetching the pchain are logged as warns.

This also closes #543 and some comments on the previous PR in the stack as well

[yacovm]

I'm not a big fan of this approach. We should ideally not go to the P-chain every time we need to query the validator set of our L1.

Ideally we'll cache the result and only update it when we know for sure that we have changed the validator set.

Since the platform chain is the first chain to bootstrap, we should have an up to date chain by the time it finished bootstrapping.

Every subsequent query should return a cached value which will be updated when there is evidence for it (we saw a finalization of a sealing block)