WIP: Update portalocker for integration in misc/io Interfaces writing files

[oesteban]

Request raised from here: #1028 (comment)

[oesteban]

@satra, portalocker is currently available in the PyPi, should we still copy it under externals?

[satra]

we could it add it to the requirements, but also copy it to externals, since it's a single file (if it still is).

[oesteban]

It's two files now (they added a utils module), not a big deal

[oesteban]

@satra, I'm currently using portalocker over NFS and two machines running the same workflow on two different datasets. Portalocker throws "Stale handler" exception. I guess this means it is not safe in NFS, am I wrong?

[satra]

@oesteban - that's how i would interpret it. nfs is notoriously bad at syncing and can take upto 60s to do so.

[oesteban]

@satra what about http://flufllock.readthedocs.org/en/latest/ ?

[chrisgorgo]

fluflock looks promising