invoker lock-file conflict on nfs cluster

When running pyopencl on a cluster with an nfs filesystem, a lock file created in my home dir on one node prevents the other nodes from progressing. I've pasted a stack trace below.

At first I thought I could fix the problem by supplying the "cache_dir" argument when creating the pyopencl context, to point to somewhere in `/tmp` which isn't in the nfs. However, those lock files aren't the problem: The problem is the use of PersistentDict to define the "invoker_cache" in invoker.py using the default lock file location, which is inside my home dir on the nfs, in my case.

As a workaround, I've modified `invoker.py` on my system so the definition
reads
```
invoker_cache = PersistentDict("pyopencl-invoker-cache-v1",
key_builder=NumpyTypesKeyBuilder(),
container_dir='/tmp/cl/invoker')
```

Perhaps in future versions of pyopencl you could make the container_dir
configurable?