Consider removing Tables.allocatecolumn in vcat

[bkamins]

Related https://julialang.zulipchat.com/#narrow/stream/274208-helpdesk-.28published.29/topic/vcat.20dataframes.20with.20pooledarrays

Currently we use Tables.allocatecolumn as it is safe. E.g. currently we have an issue with PooledArrays.jl https://github.com/JuliaData/PooledArrays.jl/issues/88 which current design handles correctly.

Probably it would be better to improve over Tables.allocatecolumn (which looks only at element type and disregards container type)