Improve SQL readability in SqlServer Query Providers

[crwells]

Motivation and Context (Why the change? What's the scenario?)

Reading the SQL in these two classes is very difficult, and there is a lot of repetitive code in each method.

Example of previous SQL block:

        var queryColumns = $"{this.GetFullTableName(this._config.MemoryTableName)}.[id]," +
                           $"{this.GetFullTableName(this._config.MemoryTableName)}.[key]," +
                           $"{this.GetFullTableName(this._config.MemoryTableName)}.[payload]," +
                           $"{this.GetFullTableName(this._config.MemoryTableName)}.[tags]";

Now becomes

        var memoryTable = this.GetFullTableName(this._config.MemoryTableName);

        var queryColumns = $"{memoryTable}.[id]," +
                           $"{memoryTable}.[key]," +
                           $"{memoryTable}.[payload]," +
                           $"{memoryTable}.[tags]";

The important part of the code (generating a SQL statement) no longer gets washed out by the repetitive, unimportant code.

High level description (Approach, Design)

This changes generates the table names at the beginning of each method and then uses them in constructing the queries. This improves readability, and makes it easier to make changes in the future.