Improve deleteat!(x,::Integer)

[laborg]

We only need to remove one element from a ChainedVector and can therefore special case the fact that if this element is a single entry in a chunk the chunk can be removed completely. This reliefs us from the slow cleanup!() call.
As a result dependent functions performance improve:

pop!() with many chunks

julia> function popall(x)
           while length(x) >0
               pop!(x)
           end
       end
popall (generic function with 1 method)

julia> popall(ChainedVector([[1]])); # compilation

julia> cv = ChainedVector([rand(Int,5) for _ in 1:20000]);

julia> @time popall(cv) # main
  1.043174 seconds

julia> @time popall(cv) # pr
  0.003701 seconds

popfirst!() with many chunks

julia> function popall(x)
           while length(x) >0
               popfirst!(x)
           end
       end
popall (generic function with 1 method)

julia> popall(ChainedVector([[1]])); # compilation

julia> cv = ChainedVector([rand(Int,5) for _ in 1:20000]);

julia> @time popall(cv) # main
  1.423213 seconds

julia> @time popall(cv) # pr
  0.128358 seconds 

popfirst!() with single chunk

julia> function popall(x)
           while length(x) >0
               popfirst!(x)
           end
       end
popall (generic function with 1 method)

julia> popall(ChainedVector([[1]])); # compilation

julia> cv = ChainedVector([rand(Int,100000)]);

julia> @time popall(cv)
  0.002024 seconds # main

julia> @time popall(cv)
  0.001135 seconds # pr