Upgrade log4net to v2.0.12

[stakx]

No description provided.

[stakx]

The .NET Core test run on Ubuntu is now failing; it appears that this is due to an issue in log4net 2.0.10's MemoryAppender. While not documented, it appears to have been resolved in 2.0.12.

We have the following options:

1. don't update log4net at all
2. update to 2.0.10 and disable the failing tests
3. update to 2.0.12, hoping that the issue really did get fixed, ad they just forgot to update the issue tracker status (vs. the bug just no longer surfacing by pure chance)

(2), i.e. picking a buggy version and skipping tests because of that, seems like the worst option to me. I'm torn between (1) and (3). If (3) is still buggy, it'd be bad to force people to use that version.

[jonorossi]

@stakx I'd go with option 3, upgrade to 2.0.12. 2.0.10 was released 5 months ago with .11 a month later and .12 another month later, close enough that it is unlikely to be a problem. NuGet will pretty much treat it as a patch version anyway, 2.0.12 nearly has 3 times the number of downloads, and we are making a major release next too.