Lots of allocations when writing DateTimes

I was having some crazy GC pauses, and tracked it down to this:
```
using Dates
using Arrow

julia> tstamps = DateTime.(zeros(1_000_000));
julia> @time Arrow.write("test.arrow", (;t=tstamps))
0.055171 seconds (1.00 M allocations: 24.274 MiB)
"test.arrow"
```
There seems to be one allocation per `DateTime`. I'm on julia 1.6.1, Arrow 1.4.1.

Thanks!

Reproduce the Julia 1.6.1 example using DateTime.(zeros(1_000_000)) and Arrow.write("test.arrow", (;t=tstamps)); then trace the DateTime serialization path used by Arrow.write. Done means writing the DateTime column without one allocation per value and confirming the allocation count with @time.

Written by the indexing model from the issue text.