tuple: zap tuple_extra

tuple_extra() allows to store arbitrary metadata inside tuples.
To use it, one should set extra_size when creating a tuple_format.
It was introduced for storing UPSERT counter or column mask inside
vinyl statements. Turned out that it wasn't really needed as UPSERT
counter can be stored on lsregion while column mask doesn't need to
be stored at all.

Actually, the whole idea of tuple_extra() is rather crooked: why
would we need it if we can inherit struct tuple instead, as we do
in case of memtx_tuple and vy_stmt? Accessing an inherited struct
is much more convenient than using tuple_extra().

So this patch gets rid of tuple_extra(). To do that, it partially
reverts the following commits:

6c0842e0 vinyl: refactor vy_stmt_alloc()
74ff46d8 vinyl: add special format for tuples with column mask
11eb7816 Add extra size to tuple_format->field_map_size