Added 1.9.1 change logs.

Conflicts:
	CSharpDriver.sln
Added release notes for 1.9.1.

Conflicts:
	CSharpDriver.sln
	Release Notes/Release Notes v1.9.1.md
CSHARP-993: Ensure unique ObjectIds across AppDomains.
CSHARP-986: Allow $external as a database name even though it contains a "$".