vinyl: don't yield in on_commit and on_rollback triggers

To apply replicated rows in parallel, we need to be able to complete
transactions asynchronously, from the tx_prio callback. We can't yield
there so we must ensure that on_commit/on_rollback triggers don't yield.
The only place where we may yield in a completion trigger is vinyl DDL,
which submits vylog records and waits for them to complete.

Actually, there's no reason to wait for vylog write to complete, as we
can handle missing records on recovery. So this patch reworks vylog to
make vy_log_tx_try_commit() and hence on_commit/on_rollback triggers
using it non-yielding.

To achieve that, we need to:

 - Use vy_log.latch only to sync log rotation vs writes. Don't protect
   vylog buffer with it. This makes vy_log_tx_begin() non-yielding.

 - Use a separate list and buffer for storing vylog records of each
   transaction. We used to share them among transactions, but without
   vy_log.latch we can't sync access to them anymore. Since vylog
   transactions are rare events, this should be fine.

 - Make vy_log_tx_try_commit() append the transaction to the list of
   pending transactions and wake up a background fiber to flush all
   pending transactions. This way it doesn't need to yield.

Closes #4218