HC-1746 - DateTimeOffset param support in SqlCE

The bug was that you could not filter on DateTimeOffset parameters when
using the SqlCe provider. When SqlCeProviderServices encountered a
DateTimeOffset parameter in a filter expression it would simply throw an
excpetion saying that it was not supported. Following the existing
pattern that we established for DateTimeOffset values in SQL CE, I
modified SqlCeProviderServices to treat PrimitiveTypeKind.DateTimeOffset
as VNarChar(40), and I modified DbProviderServices to convert
DateTimeOffset values to UTC strings when it is generating parameters for
the actual SQL query. It will only do this convertion if the value is a
DateTimeOffset and the database field is a string type. Technically this
change is not specific to SQL Compact, so it should mean that in any DB
provider if you somehow find a way to make an expression that compares a
string to a DateTimeOffset it will actually work. This is an acceptable
tradeoff which we could even argue is a feature.