Optimize GRANDPA justifications before submitting them to the pallet-bridge-grandpa

[svyatonik]

Noticed one strange thing when testing our relay with Kusama/Polkadot - within the same session, GRANDPA justifications received from the node contain different number of signatures. E.g. given session with 1000 validators in Kusama, on justification has 672 precommits, other has 674 and the next one has 676. Since on-chain signature verification is expensive, it'd be better for relayers to "optimize" justifications before submitting them. Iiuc in this (1000 validators) case we only need 667 signatures, so extra ~10 signatures may be removed before sending it to the pallet.

[acatangiu]

Noticed one strange thing when testing our relay with Kusama/Polkadot - within the same session, GRANDPA justifications received from the node contain different number of signatures.

This is normal, GRANDPA justification is built using all the available/gossiped signatures, the condition being >= 2/3+1, so they can be more than 667 for 1000 validators.

so extra ~10 signatures may be removed before sending it to the pallet

Sounds good, if it's a low-hanging fruit let's do it.