Optimize PackedFieldDescriptor to avoid Bit operations; DO_MAX_1000_COLUMNS definition

[SergeiPavlov]

• Remove PackedFieldDescriptorExtensions

short and byte fields are faster than Shift + AND bit operations

These bit operations are still used at Recordset's fields mapping time, which invoked less frequently than field access operations.

ushort Index field is enough for ~30K fields in recordset because longest Value-types (Guid, DateTimeOffset, decimal) eat 2 longs

Also:

• Remove [NonSerialized] on AccessorIndex field. It was a rudimentary from time when Accessor field was reference-typed

• Add DO_MAX_1000_COLUMNS compilation constant to pack PackedFieldDescriptorExtensions into 4 bytes instead of 8. Arrays of this struct consume significant memory. I see:
  51,630 4,911,832 Xtensive.Tuples.Packed.PackedFieldDescriptor[] in the last dump

In this mode max # of recordset columns is ~1000