xlog: fix sync_is_async xlog option

The behaviour change was introduced in cda3cb55: sync_is_async option
was forgotten to be updated from xdir; sync_interval was forgotten too,
but was restored in 1900c58b.

The commit fixes the performance regression around 6-14% for average RPS
on default nosqlbench workload with 30 seconds duration. The additional
information about benchmarking can be found in #3747.

Thanks to Vladimir Davydov (@locker) for the investigation of the
cda3cb55 changes.

Closes #3747