Write each line to the OS without an fsync

Write each line to the OS without an fsync

Each log line and each node goes to the operating system with one write
and no buffer before the call returns, so a crash of the process loses
nothing. view.json stays atomic through a temporary file and a rename.
An fsync on each write cost about 96 ms per message on a typical disk,
which made a long replay take hours. Durability against a power loss is
a later group commit, with one background sync that makes all earlier
writes durable.

Issue: beb9a70