Linq to Cargo improvements

[CXuesong]

Follow-up on #77

• Introduce support for some DB functions. See CargoPostEvaluationTranslator and CargoTests for supported functions and how to call them with C# Expressions.
  • UPPER ("string".ToUpper())
  • LOWER ("string".ToLower())
  • TRIM ("string".Trim())
  • INSTR & LOCATE ("string".Contains(), "string".IndexOf()) (Not supported by Cargo by default)
  • SUBSTRING ("string".Substring())
  • LEN ("string".Length) (Not supported by Cargo by default)
  • NOW (DateTime.Now, DateTimeOffset.Now)
  • YEAR, MONTH, DAY, DATE (dt.Year, dt.Month, dt.Day, dt.Date)
  • DATE_ADD, DATE_SUB (dt + timespan, dt.Add(timespan), dt - timespan, dt.Subtract(timespan))
  • LIKE (CargoFunctions.Like)
  • HOLDS (CargoFunctions.Holds)
  • HOLDS LIKE (CargoFunctions.HoldsLike)
  • FLOOR, CEIL, POWER, LOG, ROUND (Math.Floor, Math.Ceiling, Math.Pow, Math.Exp, Math.Log, Math.Round)
  • See mw:Extension:Cargo/Querying data#Using SQL functions for a list of DB functions that should get support eventually.
• Introduce support for deserializing Cargo list column into ICollection<T> property in data model.
• Introduce support for querying & deserializing nullable columns (Nullable<T> or T?).
  • Optionally use CargoListAttribute to customize the list behavior.
• Add CargoQueryContext.PaginationSize for adjusting internal pagination size when requesting a long sequence of record set from MediaWiki server.