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)