test SWAP locks

[mortelli]

It seems the locking scheme introduced in #1725 makes sense, and somewhat fixes the original problem in #1663.

However, we have not found a way to programmatically test this scheme yet.

[mortelli]

apologies—i told @holisticode to close this one but upon further review i think we haven't covered it yet.

it's not a high priority issue: this is about implementing (if possible) tests to verify specifically how parallel routines/threads will behave when calling accounting functionalities and using shared resources.

[mortelli]

I'm closing this issue due to:

• it being too ambitious, i.e. too hard to really test.
• it being more related to testing golang itself, rather than the incentives module.