lua/xlog: don't ignore unknown header fields

The xlog reader Lua module uses the xlog_cursor_next_row, which decodes
the row header with xrow_header_decode. The latter silently ignores any
unknown fields, which complicates catching bugs when garbage is written
to a row header by mistake, for example, see #8783.

Let's parse a row header without using xrow_header_decode in the xlog
reader module, like we parse a row body, and output all unknown/invalid
keys as is.

To do that, we have to extend the xlog cursor API with the new method
xlog_cursor_next_row_raw that returns a pointer to the position in the
tx buffer where the next xrow is stored without advancing it. To avoid
a memory leak in case the caller fails to parse an xrow returned by this
function, we also have to move the call to xlog_tx_cursor_destroy from
xlog_tx_cursor_next_row to xlog_cursor_next_tx.

While we are at it,
 - Don't raise an error if a key type encountered in a row body is
   invalid (not an integer). Instead, silently ignore such keys.
 - Remove the useless body MsgPack validness check because we already
   check it after decoding the header.
 - Add error injection based tests to check all the corner cases.

NO_DOC=bug fix