Pick proper abstractions to interoperate with kdb+ tables

[abalkin]

References

• DataStreams.jl - package aims to define a generic and performant framework for the transfer of "table-like" data.
• IterableTables.jl - A common table interface for julia
• An Update on DataFrames Future Plans (Jul 4, 2017)

[abalkin]

It appears that DataFrames is still the mainstream. It is currently being changed to use Union{T,Null} to handle missing values, so until this change is tagged, we will not do anything special about kdb+ nulls.

[abalkin]

Query.jl and IterableTables.jl have PRs to port to Nulls.jl

• IterableTables.jl - Add support for Nulls.jl based DataFrames.jl
• IterableTables.jl - Port to Nulls
• Query.jl - Port to Nulls.jl