cleanup/nullable-ize IField-derived classes

[gbirchmeier]

Finished cleaning up all the nullable-enable warnings in the "QuickFIX" csproj!

• deprecate lower-case-starting function names (renamed to upper-case-starting)
• deprecate Field.Obj (renamed to Value)
• deprecate Field.getValue/setValue (just use Value getter/setter)
• major cleanup of MessageTests.cs
• started cleaning up SessionTests.cs, but that needs more work