Simplify Sql.query_row_col function

Function `query_row` executes query and calls callback function to process
row returned. Function query_row_col() is special case, that provides
callback function, which returns value of one particular column of row,
ignoring others.

In all cases, that particular column was 0 (first and only column of
query result), since there is no point to select more than one column
with this function -- they are discarded anyway.

This commit removes that redundancy, removing column number argument of
query_row_col() function and adjusting call sites accordingly.

src/key.rs

@@ -146,7 +146,6 @@ impl Key {
             context,
             "SELECT public_key FROM keypairs WHERE addr=? AND is_default=1;",
             &[addr],
-            0,
         )
         .and_then(|blob: Vec<u8>| Self::from_slice(&blob, KeyType::Public))
     }
@@ -160,7 +159,6 @@ impl Key {
             context,
             "SELECT private_key FROM keypairs WHERE addr=? AND is_default=1;",
             &[self_addr.as_ref()],
-            0,
         )
         .and_then(|blob: Vec<u8>| Self::from_slice(&blob, KeyType::Private))
     }