Shrink SqlParameter XmlSchema

[Wraith2]

In dotnet/corefx#35549 I changed SqlParameter to use SqlMetaDataXmlSchemaCollection to hold xml schema information if it is present. This reduces the memory size because rather than keeping 3 string references which will mostly be null a single reference is kept for the sub object and it is created before use. This ports that change to netcore and netfx and adds the covering tests, it includes the fixes from dotnet/corefx#41008 which covered a problem in the original.

While working on SqlParameter i made a couple of other minor changes:

The direction field was not initialized but had a getter which checked for the 0 value and then returned ParameterDirection.Input. Recent profiling work on parameter passing showed that getting the parameter direction is an unexpectedly hot call. Because it is not possible for users to observe the 0 value it is safe to simply initialize the value in the ctors and then use a simple getter. The only observable change in this is if a user changes the value and is using INotifyPropertyChanged (usually designers use this) they will now not see a change from 0 to 1 if they assign to the Direction property, given that they could never observe the 0 value this seems like it is unlikely to break anyone.

The ctor call chain in netfx and netcore was slightly different so I synchronized them. This should have no practical effect but it will make merging the files later easier.

I removed some uses of ADP.StrEmpty and replaced them with string.Empty, these two expressions point to the same string instance because of runtime interning of constant strings. Using our own alias for it is no more secure than referring to it as string.Empty or "", as such just use the runtime supported constant and remove our copy from the library at some future date when all other references have been removed.