region_reset does not reset usage to 0 as expected

[nshy]

As a result region_join(&region, 0) after reset will prepend garbage left in region slabs other than the first one. In parcular this is what src/http.c and src/lua/http.c of Tarantool 3.0.0-alpha1-16-g0e5a3cc21 do for region with headers.

There is a patch 67d7ab4 which seems to fix the issue but it does not make it into upstream.