vinyl: don't sync WAL on space alter if not necessary

Changes done to an altered space while a new index is being built or
the format is being checked are propagated via an on_replace trigger.
The problem is there may be transactions that started before the alter
request. Their working set can't be checked so we simply abort them.
We can't abort transactions that have reached WAL so we also call
wal_sync() to flush all pending WAL requests. This is a yielding
operation and we call it even if there's no transactions that need
to be flushed. As a result, vinyl space alter yields unconditionally,
even if the space is empty and there is no pending transactions
affecting it. This prevents us from implementing transactional DDL.
Let's call wal_sync() only if there's actually at least one pending
transaction affecting the altered space and waiting for WAL.