Make `splice` configurable

In line with the standard Swift implementation.

For reference, some performance numbers (all running in VMs, so no
absolute numbers at all), using a 128MB object:

Without `splice`:
```
$ for i in `seq 1 20`; do swift download  testing 128M; done
128M [auth 0.193s, headers 0.593s, total 1.318s, 119.395 MB/s]
128M [auth 0.191s, headers 0.437s, total 1.335s, 117.347 MB/s]
128M [auth 0.162s, headers 0.407s, total 1.175s, 132.514 MB/s]
128M [auth 0.171s, headers 0.430s, total 1.303s, 118.625 MB/s]
128M [auth 0.164s, headers 0.426s, total 1.190s, 130.750 MB/s]
128M [auth 0.187s, headers 0.440s, total 1.194s, 133.355 MB/s]
128M [auth 0.171s, headers 0.434s, total 1.215s, 128.572 MB/s]
128M [auth 0.181s, headers 0.450s, total 1.271s, 123.144 MB/s]
128M [auth 0.172s, headers 0.408s, total 1.315s, 117.486 MB/s]
128M [auth 0.167s, headers 0.382s, total 1.219s, 127.526 MB/s]
128M [auth 0.181s, headers 0.421s, total 1.254s, 125.074 MB/s]
128M [auth 0.177s, headers 0.423s, total 1.173s, 134.765 MB/s]
128M [auth 0.193s, headers 0.492s, total 1.196s, 133.795 MB/s]
128M [auth 0.202s, headers 0.464s, total 1.310s, 121.195 MB/s]
128M [auth 0.168s, headers 0.544s, total 1.281s, 120.598 MB/s]
128M [auth 0.170s, headers 0.446s, total 1.302s, 118.547 MB/s]
128M [auth 0.163s, headers 0.419s, total 1.111s, 141.657 MB/s]
128M [auth 0.168s, headers 0.440s, total 1.323s, 116.195 MB/s]
128M [auth 0.160s, headers 0.412s, total 1.174s, 132.366 MB/s]
128M [auth 0.166s, headers 0.420s, total 1.266s, 122.045 MB/s]
```

Using `splice`:
```
$ for i in `seq 1 20`; do swift download  testing 128M; done
128M [auth 0.165s, headers 0.514s, total 1.154s, 135.751 MB/s]
128M [auth 0.161s, headers 0.403s, total 1.025s, 155.484 MB/s]
128M [auth 0.209s, headers 0.473s, total 1.031s, 163.219 MB/s]
128M [auth 0.188s, headers 0.506s, total 1.203s, 132.231 MB/s]
128M [auth 0.185s, headers 0.476s, total 1.186s, 133.994 MB/s]
128M [auth 0.180s, headers 0.549s, total 1.284s, 121.618 MB/s]
128M [auth 0.164s, headers 0.426s, total 1.068s, 148.417 MB/s]
128M [auth 0.216s, headers 0.579s, total 1.216s, 134.293 MB/s]
128M [auth 0.198s, headers 0.461s, total 1.008s, 165.718 MB/s]
128M [auth 0.170s, headers 0.411s, total 1.068s, 149.526 MB/s]
128M [auth 0.170s, headers 0.494s, total 1.131s, 139.728 MB/s]
128M [auth 0.179s, headers 0.432s, total 1.081s, 148.786 MB/s]
128M [auth 0.207s, headers 0.546s, total 1.378s, 114.566 MB/s]
128M [auth 0.168s, headers 0.439s, total 1.168s, 134.186 MB/s]
128M [auth 0.162s, headers 0.408s, total 1.036s, 153.662 MB/s]
128M [auth 0.185s, headers 0.428s, total 1.123s, 143.089 MB/s]
128M [auth 0.164s, headers 0.422s, total 1.059s, 149.994 MB/s]
128M [auth 0.185s, headers 0.441s, total 1.160s, 137.688 MB/s]
128M [auth 0.165s, headers 0.482s, total 1.075s, 147.596 MB/s]
128M [auth 0.165s, headers 0.416s, total 1.162s, 134.653 MB/s]
```

Next to the moderate throughput impact, there was a clearly visible CPU
usage difference of the `swift-object-server` processes in `top`, going
from 30-50% to 10% and less.