Version overwrite on simultaneous write of same version

[jgerardsimcock]

If two users are trying to write to the same archive at the same time, two versions with the same version number will be uploaded to the manager. These two versions will point to the same path in the fs and the last closed file will be the object associated with that version.

{'_id': 'big climate data', 'versions': [ '1.0.0', 'dependencies': [('arch1','1.0.1')], '1.1.0', 'dependencies': [('arch1', '1.1.0'), ('arch2', '1.0.0'), '1.1.0', 'dependencies': [('arch1','1.0.1')]

In this case, the manager will reflect both version 1.1.0 entries but datafs will pull the last entry, not reflecting the earlier dependency update.

[delgadom]

Just as a reminder of our discussion, rather than failing (succeeding?) silently, this should raise an error for the second user on write. e.g.

User 1                            | User 2
-------                           | -------
                                  |
api1 = get_api()                  |
                                  | api2 = get_api()
arch = api1.get_archive('test')   |
                                  | arch = api2.get_archive('test')
with arch.open('w+') as f:        |
                                  | with arch.open('w+') as f:
  f.write('hello 1')              |
                                  |   f.write('hello 2')              
                                  |
                                  | Traceback (most recent call last):
                                  | ...
                                  | IOError: Simultaneous write occurred. User 
                                  | <username> uploaded version <ver> at 
                                  | <timestamp>