LIU-368: Enable Drop persistence

[myxie]

Problem
This PR partially addresses LIU-368, which aims to enable proper persistence of data drops.

Currently, the persist and expireAfterUse flag are coupled together. This is orthogonal to the original behaviour, in which the expiry of the drop data should have no bearing on the persistence of the data (which would ensure it is copied elsewhere).

This PR partially addresses this work by de-coupling the expireAfterUse behaviour from the persist behaviour.

Solution

• Remove persist check in dlm.expireCompletedDrops
• Updated the default values in dlg.drop so that if we only use the lifespan if we don't specify expireAfterUse in the Drop arguments. Otherwise, we would set expireAfterUse to False and then the lifespan would be set, which is the opposite of what we want.

The current default behaviour exists:

• FileDrop: expireAfterUse=False
• MemoryDrop: ExpireAfterUse=True
• Other Drops: ExpireAfterUse=None
  • If "lifespan" != -1, then the drop will still expire (after the timeout)
  • Else, the drop will not expire after use.

Testing

• I've confirmed using a simple Hello World graph that setting the expireAfterUse to True will lead to the file.out being removed from the workspace; setting it to False leaves it present.
• I've updated the test cases to be more explicit about the default behaviours for both File and Memory drops.

Future work
The 'persist' option has not been completely fulfilled. More work needs to be done to:

• Add a file-store if the 'persist' option is selected
• Establish a warning for persist when there is no persistent file-store specified.
• Pickling/serialization mechansim for a MemoryDrop that is marked as persistent
• Ability to load serialized MemoryDrops