CSHARP-980:fixed issue with Kerberos corrupting managed memory.
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 $external as a database name even though it contains a "$".
CSHARP-987: fixed issue with bulk upsert identifiers on pre-2.6 servers.
CSHARP-987: Code review changes.
CSHARP-995: Add support for reading the new extended JSON formats added in server 2.6.
CSHARP-995: Adapt to master branch.
rebase