journal performance improvements - use asynchronous I/O
Currently journal I/O is done synchronously, including fsync(). fsync
is very slow (see #3556) and we probably cannot get rid of it completely, so I think we should use asynchronous I/O for journal processing. That would allow the server to process normal queries while waiting for journal operations.
TODO: Test IXFR-in and querying the server in parallel.