Work towards merging SqlDataReader

[MichelZ]

Same as #2953 but for SqlDataReader

• Comments Align comments between netfx and netcore codebases #2966
• Port IDBColumnSchemaGenerator to netf Merge | Add IDBColumnSchemaGenerator interface to netfx SqlDataReader #2967
• Align Constrained Execution Region with netcore for merging netcore/netfx #2969

[edwardneal]

Besides the coding style/comments/CERs, there's other work:

• Complete port of PR 521 to netfx #2964
• Port perf: Move datareader caches down to internal connection #499 to netfx
• Consider implementing IDbColumnSchemaGenerator. I've noticed this absence and noticed that the interface exists in .NET Standard 2.0 and .NET Framework 4.7.1. We're building for .NET Framework 4.6.2, and I've not checked how the dependencies will shake out here

[edwardneal]

After merging #2967, the diff between netfx and netcore's SqlDataReader implementation is now -407/+159. Most of this is now taken up by two points:

• Porting of perf: Move datareader caches down to internal connection #499
• Reinstate the exception handling for OutOfMemoryException, StackOverflowException and ThreadAbortException. None of these have any effect on netcore

#3042 is a notable style change though. Once that's merged, I can port the exception handling across as time permits (unless someone else wants to pick this up sooner.) I'll submit a port of PR 499 (and any remaining dependencies/post-PR changes.)

The rest of the differences are just differences in logging text and debug assertions, along with ~10 lines of behavioural changes in TrySetMetaData and IsDBNull.