Check content before zip/unzip the txhashset (#1174)

* Check txhashset content before zip/unzip
* Add header in txhashset verification
* Add copy function and test
* Add file util
* Now check and remove unexpected files instead of just crashing

core/src/core/pmmr.rs

@@ -285,7 +285,7 @@ where
 		Ok(elmt_pos)
 	}
 
-	/// Saves a snaphost of the MMR tagged with the block hash.
+	/// Saves a snapshot of the MMR tagged with the block hash.
 	/// Specifically - snapshots the utxo file as we need this rewound before
 	/// sending the txhashset zip file to another node for fast-sync.
 	pub fn snapshot(&mut self, header: &BlockHeader) -> Result<(), String> {