vinyl: get rid of vy_env::join_lsn

This fake LSN counter, which is used for assigning LSNs to Vinyl
statements during the initial join stage, was introduced a long time
ago, when LSNs were used as identifiers for lsregion allocations and
hence were supposed to grow strictly monotonically with each new
transaction. Later on, they were reused for assigning unique LSNs to
identify indexes in vylog.

These days, we don't need initial join LSNs to be unique, as we switched
to generations for lsregion allocations while in vylog we now use LSNs
only as an incarnation counter, not as a unique identifier. That said,
let's zap vy_env::join_lsn and simply assign 0 to all statements
received during the initial join stage.

To achieve that, we just need to relax an assertion in vy_tx_commit()
and remove the assumption that an LSN can't be zero in the write
iterator implementation.