contrib: Use batched JSON-RPC in linearize.py for speedup

For what it's worth;
```
$ time python linearize-hashes.py linearize.cfg > hashlist.txt.1
real    4m47.909s
user    2m45.033s
sys     0m49.987s

$ time python linearize-hashes-new.py linearize.cfg > hashlist.txt.2
real    0m9.487s
user    0m2.721s
sys     0m0.363s

$  sha256sum hashlist.txt*
e89d542fb65666eeb63c0a74995e7481b8349d9b48f9139f3ef3056c462837ab  hashlist.txt.1
e89d542fb65666eeb63c0a74995e7481b8349d9b48f9139f3ef3056c462837ab  hashlist.txt.2
```
(speedup about 30×)