vinyl: refactor vy_branch_write()

* Merge vy_branch_update_stat() into vy_branch_write()
* Refactor firstvalue/lastvalue logic in vy_branch_write()
* memcpy() entire tuple instead of copying field by field
* Remove sf_comparable_size() and sf_comparable_write()
* Add missing vy_buf_free(&compressed)

Needed for #1594.