Description: In the latest installment of "Ben Tries To Combine Two Projects Into One", we are merging the SqlTransaction class. This is actually a pretty easy merge - there are ~4 methods that need to be merged into a single file in the common project.

The big headache with these are that they use the constrained execution region pattern that only applies to netfx - this means each of these methods have an entirely different set of exception handling for special exceptions that can only happen in netfx constrained execution regions. However, the netcore implementation has a diagnostic listener that expects an exception to be set on it if one occurs. This presents a challenge of how to retain these differences while also making it readable. There are two patterns we could adopt:

1. All exception handles have a conditional for netcore to set the diagnostic listener eg:

//...catch(OutOfMemoryExceptione){  #ifNETdiagnosticListener.SetException(e)  #endif  _connection.Abort();throw;}catch(ThreadAbortExceptione){// .. repeat from above}catch(Exceptione){  #ifNETdiagnosticListener.SetException(e);  #endifthrow;}

2. Conditional exception handles for netfx/netcore

#ifNETFRAMEWORKcatch(OutOfMemoryExceptione){_connection.Abort();throw;}catch(ThreadAbortExceptione){// .. repeat from above}#elsecatch(Exceptione){diagnosticListener.SetException(e);throw;}#endif

In my humble opinion, option 2 is somewhat better, and it is the pattern I have adopted in this PR.

The other merge as part of this PR is moving SqlTransaction.Common back into the SqlTransaction file. Thus, no more partial for SqlTransaction. The Common file looks to be an early attempt to merge what could be merged, so since we have reconciled the differences, we can bring them all together again.

Testing: Once again, no functional differences, just code moving around.

@edwardneal Please don't be upset if I sniped your next merge target 😄