Support Row with large values.

Motivation.

For now, TupleAssembler writes offsets for varlen columns as 2-byte {{short }}type.
This implicitly restricts key/value sizes down to 64 kB in total.
On another side, for small rows that columns can be addressed with byte type, we will waste few bytes.

Description.

Let's

1. allow 4 byte types (byte, short, int) for offsets.
2. implement and benchmark different approaches that allow us to write rows in the most compact way.
3. then choose and merge the best one.

We can introduce several formats for writing Vartable (using byte/short/int offsets).
Additional information about Vartable format can be coded into chunk flags.

The first approach is to precalculate chunk total size, then choose the most compact format and write a chunk.
The second approach is to write a chunk with the widest format then convert the chunk into the most compact format in place.