Rewind head and header_head consistently. (#2918)

* maintain header_head as distinctly separate from head

* cleanup corrupted storage log msg

* simplify process_header and check_header_known

* remember to commit the batch when successfully processing a header...

* rework sync_block_headers for consistency with process_block_header

* cleanup unrelated code

* fix pool tests

* cleanup chain tests

* cleanup chain tests (reuse helpers more)

* cleanup - head not header on an extension
shortcircuit "rewind and apply fork" for headers if next header

store/src/types.rs

@@ -109,13 +109,7 @@ where
 	pub fn read(&self, position: u64) -> Option<T> {
 		match self.file.read_as_elmt(position - 1) {
 			Ok(x) => Some(x),
-			Err(e) => {
-				error!(
-					"Corrupted storage, could not read an entry from data file: {:?}",
-					e
-				);
-				None
-			}
+			Err(_) => None,
 		}
 	}