Reduce cloning in GrandpaBlockImport

[andresilva]

When importing or finalizing a block through GrandpaBlockImport it currently eagerly clones the existing authority_set in case any intermediary operation fails and it is necessary to revert.

substrate/core/finality-grandpa/src/lib.rs

Lines 799 to 800 in ea3d06a

	// TODO [andre]: clone only when changed
	let old_authority_set = authority_set.clone();