InMemoryChannelLayer improvements, test fixes

[devkral]

Improvements to InMemoryChannelLayer

• honor expiry / don't block for eternity
• honor per channel capacities
• threadsafer cleaning design with atomic operations
• parallel sending

Bugfixes tests

• channel_layer is sometimes overwritten and the tests become bogus

[devkral]

Note: I made an mistake with:

• honor expiry / don't block for eternity
  This is the correct behavior for receive.
  But maybe the receive method it should be put in a loop, in case the queue is expired
  (see devkral/channels-multiprocessing for an example)

[bigfootjon]

Thanks for the quick reply, sorry I'm just getting back to you.

This looks reasonable to me, I just have a few questions so I'm marking it as requested changes.