Is your feature request related to a problem? Please describe.

When our application is starting it loads a lot of data from DB and it causes memory consumption growth and high pressure to GC because of big memory traffic.
On the third place of killed objects is boxed System.Data.SqlTypes.SqlGuid struct.
These boxed values are 99.9% created when TdsParser is putting SqlGuid struct to SqlBuffer.SqlGuid property (see the image bellow).

Describe the solution you'd like

SqlBuffer already supports an efficient way to store Guid values. What I would like to do is:

• change implementation of Microsoft.Data.SqlClient.SqlBuffer
  • remove SqlGuid value from StorageType enum (taking the second alternative approach, removing of SqlGuid value from enum looks like a bad idea);
  • save Guid representation of SqlGuid to _value._guid (similar to what Guid setter does, but with checking if SqlGuid value is null)
  • read SqlGuid value from _value._guid

Describe alternatives you've considered

1. add a field of SqlGuid type to SqlBuffer, but it seems memory inefficient
2. same as the main approach, but without removing SqlGuid value from StorageType enum, in case it is really important. The only place where it looks important to me is s_dbTypeToStorageType mapping array in Microsoft.Data.SqlClient.Server.ValueUtilsSmi, and I am not sure if it is safe to change the mapping to SqlBuffer.StorageType.Guid. It seems like it is not safe, because implementation of SqlBuffer.Guid does not support nulls.

Additional context

• Microsoft.Data.SqlClient version is 5.1.0;
• We use Microsoft.Data.SqlClient together with NHibernate;
• Almost all our db tables contain Guid columns, so, memory traffic is really high in our case.