cleanup SqlDataRecord

[Wraith2]

Follow-up from #1024 , This implements the suggested and agreed changes from that PR feedback.

1. remove EnsureSubclassOverride because there is no way to create an SqlDataRecord without the internal buffer that it is checking being set.
2. Ensure that all callsites with an ordinal parameter directly or indirectly (through GetMetaData or GetSmiMetaData) call ThrowIfInvalidOrdinal
3. remove the unused internal ctor
4. mark most fields as readonly

[TrayanZapryanov]

@Wraith2 I've looked at netcore SqlDataRecord class and found several interesting things:

1. Several things looks unused
   
2. The code in netcore SmiEventSink_Default class is quite strange - at least for me :) . I can't understand who will fill _errors and _warnings fields. If they are not used , maybe it is worth creating a static dummy class which does nothing in ProcessMessagesAndThrow.
3. _usesStringStorageForXml seems to have a constant "true" value

[Wraith2]

1. Yes, to the compiler in netcore builds. They might be used directly in the netfx build or used through reflection or inheritance. My current focus is on merging as much of the netcore and netfx codebases as possible and there are many peculiar or nonsensical things that will need resolving once that first task is finished.
2. Yes, as far as I can tell it's never used and when using SqlDataRecord the message sink functions turn up in profiles. Again, with deeper investigation on both codebases they may be possible to dummy out as you said, I just haven't tried to go deep yet. If you want to do so it would be welcome.
3. check if it's ever used in netfx, if not and there is no path to set it through inheritance then we could get rid of it.