vinyl: save 8 bytes in struct vy_stmt

Due to C/C++ standard layout sizeof(struct vy_stmt) was 32 bytes.
Is a pity since it has only 20 bytes of payload (10 byte for base
struct tuple and 10 for lsn (8) + type (1) + flags (1)).

Repack struct vy_stmt to be 24 bytes long.

Part of #5385