Threaded write on windows produces a stack overflow

For example:

```julia
using AzStorage

c = AzContainer("foo";storageaccount="myaccount")
nthreads = 2
N = round(Int, AzStorage._MINBYTES_PER_BLOCK * nthreads * 5 * (1+rand()) / 8)
nblks = AzStorage.nblocks(nthreads, N*8)
bytes_per_block = div(8*N, nblks) + (rem(8*N, nblks) == 0 ? 0 : 1)

x = rand(N)
write(c, "bar", x)
```

produces:

```
ERROR: stackOverflowError:
```