chunk requests to avoid overrunning URL length limits

With many nodes in a series, we were running into jetty URL length limits, even when pushing the limit to ridiculous lengths like 32k.  This patch chunks the request up into multiple requests, limiting the (configurable) length.  It defaults to just under 8k, which should work with most web servers.

Signed-off-by: Stephen Gran <stephen.gran@piksel.com>
Aren't you missing `f` from this iteration? Instead I'd do something like this (untested though)

```python
tmp = []
r = 0
for f in nodelist:
    tmp.append(f)
    r += 6 + len(str(f))
    if r > length:
        yield tmp
        tmp = []
        r = 0
if tmp:
    yield tmp
```