Include the epoch index in the CompactBlock as well as the block height.

[hdevalence]

Is your feature request related to a problem? Please describe.

Computing a nullifier for a state fragment requires knowing the position of the fragment. Client implementations need to compute nullifiers of state fragments as they're detected, since nullifiers are used to detect when those state fragments are consumed.

However, our current compact block definition doesn't allow computing the position of the fragments it contains without access to other state (namely, a synchronized SCT instance). Why? The position has three components: (commitment index, block index, epoch index). The commitment index is implicitly specified by the order of the fragment within the message. The block index is just the height. But the epoch index is not included in the compact block, so a client can only get it by cross-referencing with their SCT instance.

If we include the epoch index in the CompactBlock, however, we can statelessly compute positions of any fragment contained in the compact block.

Describe the solution you'd like

Add the epoch index to the compact block with a low (sub-15) field number (to save space, since it will always be present).

[hdevalence]

This would have been good to not lose track of and include in the Testnet 65 changes. Oops!

We should still consider doing this, though.

[conorsch]

This is consensus-breaking, so labeled as such. We're planning to release this week, as soon as tomorrow (#4087). If it lands before then, great.