Remove direct coupling with IConfiguration

Read all options in IConfiguration stored under key "EntityFramework:ContextName" in DbContextOptions.
Get ConnectionString & SqlServer:MaxBatchSize from DbContextOptions removing coupling with IConfiguration.
This needs a CanBeNull or a NotNull attribute. Did the tests pass without it? if so, there is probably a bug in the API consistency checker. I imagine that CanBeNull is appropriate here assuming that null means that the default batch size should be used.