Create Dragonboat log implementation that doesn't fsync

[purplefox]

Dragonboat currently fsyncs on every raft commit. This can make latencies high, especially when running tests on a Macbook.

For test runs, it would be nice if we could turn off fsync to make the testsuite run faster. This is currently not possible with the default Dragon log implementation, but we could copy the implementation into our own source tree and adapt it to turn off fsync (this is pretty trivial).

We should also investigate whether we can turn off fsync for real. See this discussion for more details:

https://github.com/lni/dragonboat/issues/201