Make SmartFormatter.Format(...) methods thread-safe

[axunonb]

• Removed ThreadStatic attribute from the Smart.Default instance of SmartFormatter.
• Added Parallel unit tests ensuring thread-safe operations with shared SmartFormatter instances with different Smart.Extensions.
• Updated documentation in Parser.cs, Smart.cs, and SmartFormatter.cs to clarify thread safety of methods.

[axunonb]

@karljj1 After making the Parser.ParseFormat method thread-safe, the same was possible for the SmartFormatter.Format methods. Of course all extensions must be thread-safe as well. For ours this is the case.

The ThreadStatic attribute for the Smart.Default instance of SmartFormatter has never been popular on users's side. When used in a multi-threaded environment like AspNet Core this was unhandy and increased GC pressure.

Removing the ThreadStatic attribute may mostly be welcome, but it is also a change in the behavior.
So do you think mentioning this change in the release notes would be sufficient?

[karljj1]

Yeah I think mentioning it should be fine. Do you know what difference this makes to performance? When I first started using the library I noticed removing the multi-threading support improved the performance. Would it be possible to put the thread support behind an ifdef so it can be toggled on/off?

[axunonb]

Thanks for your quick reply. So then the PR is ready for review.

Making the Parser thread-safe had almost no impact on performance, while at the same time we need less Parser instances.

Regarding SmartFormatter thread-safety for the Format method overloads, this was already built-in for quite a while. The only precondition for multi-threading is to set SmartSettings.IsThreadSafeMode = true, which is the default.

And yes, SmartSettings.IsThreadSafeMode = false while working on a single thread brings a nice improvement for speed but also for GC pressure. Now, that Parser.ParseFormat and Formatter.Formats are thread-safe, the penalty for thread-safety gets compensated by less Parser and SmartFormatter instances needed.

[karljj1]

Thanks! Sounds great :)