Mention issue with more than one thread and reduce workers

Due to how it appears the filesystem transport in kombu is
not thread-safe we will work around this by not having more
than one worker active at the same time in this example.

Oddly it appears the memory transport is unaffected (but
from looking at the code it doesn't look safe either), this
may just be due to how the python memory model works though.

Part of blueprint more-examples

Change-Id: Idaf04fb1a6a622af292511bbcf25329c9a5aab53
Merge "Mention issue with more than one thread and reduce workers"