Potentially fixes U4-5069 Deadlock occurring in MemberService.GetByUsername

Conflicts:
	src/Umbraco.Web/Security/MembershipHelper.cs
Ensures no membership user writing when checking public access
ignores failing test - need stephane to look
ignores log4net tests
Merge remote-tracking branch 'origin/6.2.2' into 7.1.5

Conflicts:
	src/Umbraco.Web/Security/MembershipHelper.cs