Automatically downgrade to BFT blocks when oracle usage is detected

[jvff]

Motivation

When proposing a block in which an application contract uses oracles, the block must not be a fast block. This is in order to prevent blocking the chain if the oracle responses different between validators. However, right now the client will simply reject those blocks, instead of proposing them using BFT consensus instead of the fast block consensus.

Proposal

If oracle usage is detected in the block, change it to be proposed using a BFT round.