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.
It would be good to not re-parse this every time that a new batch is created. Could you look at putting MaxBatchSize into SqlServerOptionsExtensions? We could also look into whether RelationalOptionsExtensions can be used such that any relational provider can set a match batch size, although each provider should still be able to set its own batch size.