xrow: improve corrupted header logging on an error

Improve row printing to log. Since say only has 16k buffer, there is no
point in printing the whole packet, which can have arbitrary length, in one
go.
So, print the header row by row, 16 bytes in a row, and format output to
match `xxd` output:
```
[001] 2019-04-05 18:22:46.679 [11859] iproto V> Got a corrupted row:
[001] 2019-04-05 18:22:46.679 [11859] iproto V> 00000000: A3 02 D6 5A E4 D9 E7 68 A1 53 8D 53 60 5F 20 3F
[001] 2019-04-05 18:22:46.679 [11859] iproto V> 00000010: D8 E2 D6 E2 A3 02 D6 5A E4 D9 E7 68 A1 53 8D 53
```
Now we can get rid of malloc, and use a preallocated tt_static_buf
instead.
Also, replace a big macro with a small macro and a helper function.

Followup to f645119f