make accounts_db own the directory paths (#5230)

* change paths to something accounts_db (the singleton) owns, fixes SIGILL

* fail deserialize if paths don't work
serialize paths, too

* test that paths are populated from a bank snapshot

runtime/src/bank.rs

@@ -2750,10 +2750,7 @@ mod tests {
         let mut rdr = Cursor::new(&buf[..]);
         let mut dbank: Bank = deserialize_from(&mut rdr).unwrap();
         let mut reader = BufReader::new(&buf[rdr.position() as usize..]);
-        dbank.set_bank_rc(
-            &BankRc::new(Some(bank0.accounts().paths.clone()), 0),
-            &StatusCacheRc::default(),
-        );
+        dbank.set_bank_rc(&BankRc::new(None, 0), &StatusCacheRc::default());
         assert!(dbank.rc.update_from_stream(&mut reader).is_ok());
         assert_eq!(dbank.get_balance(&key.pubkey()), 10);
         bank.compare_bank(&dbank);