speed up readuntil(IOBuffer) a bit by copying while searching for delim

ref #7066
ref https://github.com/JuliaStats/DataFrames.jl/issues/609#issuecomment-43415671

This doesn't get us all the way there but every bit helps. Note that IOStream
is still a bit faster than AsyncStream+IOBuffer. The former is used for files,
the latter for pipes.