Flush to disk option

[nblumhardt]

Fixes #18 (rolling file support needed downstream).

Log.Logger = new LoggerConfiguration()
      .WriteTo.File("log.txt", flushToDiskInterval: TimeSpan.FromSeconds(1))
      .CreateLogger();

Attempts to optimize things a bit so no flush is performed unless events have been written.

The IFlushableFileSink and PeriodicFlushToDiskSink<T> are (reluctantly) public, as this will permit the downstream RollingFileSink to wire up to the same mechanism, without us having to encapsulate the periodic flushing behavior in FileSink and SharedFileSink directly. In the long run, we might get some kind of generialized IFlushable into the base Serilog package and make it available across the board through some kind of generic wrapper.

I verified by hand that the FlushToDisk() is called, but can't come up with an easy way to test it without some kind of multi-process runner. If we query the file size from Windows from the unit test process, it'll always report a consistent view of the world to us :-).

Bonus: discovered the shared file sink tests were inadvertently ignored 😢; after enabling, found a bug preventing size limiting from taking effect. Fixed this too.

[nblumhardt]

CC @pakrym

[pakrym]

Why generic? To avoid interface method invocation?

[nblumhardt]

Yeah, I'm not so keen on this upon second thoughts. It is to make clear that the passed-in ILogEventSink should also be IFlushableFileSink (no union types in C#) but the code would be a bit nicer to read if non-generic and tested at runtime. Going to change this.

[pakrym]

LGTM

[nblumhardt]

Okay, I'm going to make a couple of minor tweaks this morning, then push through a -dev version and follow up with the RollingFile version. Should be all through as pre-release later today.