Refactor BytesToDateTimeConverter, so it works out-of-the-box with compiled models.

[lauxjpn]

Our previous BytesToDateTimeConverter implementation used methods calls in its convert to/from expressions that used a variable that was supplied to the convert expression. That is not supported by EF Core's LinqToCSharpSyntaxTranslator.VisitInvocation() implementation (would be trivial to implement though), which assumes that an InvocationExpression always contains a lambda - which is not the case.

We also accessed a private static field, which cannot be referenced from outside the class. Since the compiled model code generator does not create an instance of the value converter in question by default, but instead creates a general value converter that just uses the same expressions that we define in ours, no private fields could be accessed.

The docs state:

Conversions are defined using two Func expression trees: one from ModelClrType to ProviderClrType and the other from ProviderClrType to ModelClrType. Expression trees are used so that they can be compiled into the database access delegate for efficient conversions. The expression tree may contain a simple call to a conversion method for complex conversions.

While this seems to be just a suggestion, it seems to be a requirement for value converters in compiled models.

Fixes #1846