BlockIterator could cause UB

Blocks #37
Relate to #24

Lines 181 to 185 in 8a87a80

    
           let mut block_iter = self.block_reader(cmp, data_block_handle, options)?; 
        
           block_iter.seek(key); 
        
           if block_iter.valid() { 
        
               return Ok(Some((block_iter.key(), block_iter.value()))); 
        
           }

As block_iter.key() returns an owned Slice which represents the key field in block_iter, UB occurs after block_iter is dropped.

Temporarily solved by lengthening the lifetime of block_iter

	let mut block_iter = self.block_reader(cmp, data_block_handle, options)?;
	block_iter.seek(key);
	if block_iter.valid() {
	return Ok(Some((block_iter.key(), block_iter.value())));
	}