txhashset: remove unnecessary mutability from self references in getters (#2844)

* txhashset.rs: make self references immutable in getters

* rustfmt

chain/src/txhashset/txhashset.rs

@@ -642,7 +642,7 @@ impl<'a> HeaderExtension<'a> {
 	/// Get the header at the specified height based on the current state of the header extension.
 	/// Derives the MMR pos from the height (insertion index) and retrieves the header hash.
 	/// Looks the header up in the db by hash.
-	pub fn get_header_by_height(&mut self, height: u64) -> Result<BlockHeader, Error> {
+	pub fn get_header_by_height(&self, height: u64) -> Result<BlockHeader, Error> {
 		let pos = pmmr::insertion_to_pmmr_index(height + 1);
 		if let Some(hash) = self.get_header_hash(pos) {
 			let header = self.batch.get_block_header(&hash)?;
@@ -654,7 +654,7 @@ impl<'a> HeaderExtension<'a> {
 
 	/// Compares the provided header to the header in the header MMR at that height.
 	/// If these match we know the header is on the current chain.
-	pub fn is_on_current_chain(&mut self, header: &BlockHeader) -> Result<(), Error> {
+	pub fn is_on_current_chain(&self, header: &BlockHeader) -> Result<(), Error> {
 		let chain_header = self.get_header_by_height(header.height)?;
 		if chain_header.hash() == header.hash() {
 			Ok(())
@@ -949,7 +949,9 @@ impl<'a> Extension<'a> {
 			}
 
 			if output_pos != rproof_pos {
-				return Err(ErrorKind::Other(format!("output vs rproof MMRs different pos")).into());
+				return Err(
+					ErrorKind::Other(format!("output vs rproof MMRs different pos")).into(),
+				);
 			}
 		}
 
@@ -991,7 +993,7 @@ impl<'a> Extension<'a> {
 
 	/// Compares the provided header to the header in the header MMR at that height.
 	/// If these match we know the header is on the current chain.
-	pub fn is_on_current_chain(&mut self, header: &BlockHeader) -> Result<(), Error> {
+	pub fn is_on_current_chain(&self, header: &BlockHeader) -> Result<(), Error> {
 		let chain_header = self.get_header_by_height(header.height)?;
 		if chain_header.hash() == header.hash() {
 			Ok(())