Deprecate generic seekend method

[nhz2]

Ref: #183

[mkitti]

Use @deprecate.

help?> @deprecate
  @deprecate old new [export_old=true]

  Deprecate method old and specify the
  replacement call new, defining a new method
  old with the specified signature in the
  process.

  To prevent old from being exported, set
  export_old to false.

[nhz2]

I'm not sure what new should be in this case.

[nhz2]

Maybe it would be better not to deprecate and just remove the method in 0.11

I think the main reason for removing seekend is it enables things like:
Try to seekend and if that works use a seeking strategy, otherwise, use a streaming strategy.

Also related JuliaLang/julia#24242

[mkitti]

Well it would nice to make this compatible with --depwarn=true which should default to false for normal users.