Make DependencyTableStore a real singleton

[lmolkova]

Fixes beta3 memory issue #554

[dnduffy]

I don't recall, does C# suffer from problems with doubled checked locking the way that Java does?
Is there a reason not to just create the instance in the declaration and let the class loader handle it?

[lmolkova]

C# needs doubled checking, see https://msdn.microsoft.com/en-us/library/ff650316.aspx

I do not know why initially this was implemented as singleton. If you say it's not needed, I'm fine to initialize instance once in ctor

[dnduffy]

I'm not questioning the singleton, only the doubled checked locking for initialization instead of a static constructor call with the variable declaration.

[lmolkova]

Double checking is described in that article. Anyway I'm fine with @cijothomas PR #559. We should have it in the stable release

[dnduffy]

I would like changes but I'm going to merge this for now since I realize I merged a dependent PR and likely broke the build...

[dnduffy]

My apologies, the instance property already existed and I didn't need to panic merge this.
Sorry

[cijothomas]

This solves memory leak, but introduces a lock. Since this method is called multiple times within tracking of a single dependency, this can be perf overhead. i will slightly tweak this and submit new PR on top of this.

[lmolkova]

This solves memory leak, but introduces a lock. Since this method is called multiple times within tracking of a single dependency, this can be perf overhead.

I did not get this.

The first time Instance is called in DependencyTrackingTelemetryModule.InitializeForDiagnosticAndFrameworkEventSource it will create a new table. Even if there is some concurrency in creation, there will be no dead lock, just a lock during initialization.

After that Instance will always be there and there will be no lock at all. This is canonical multi-threaded singleton implementation.

[cijothomas]

Ignore what i said about lock- lock is not involved except for 1st time.