Split performance/throughput tests out; break dblogger circular dependency.

Rename kvlayer_throughput_test to just kvlayer_performance, and move
(py.test) kvlayer/tests/test_interface.py to (standalone)
kvlayer/tests/performance.py.  Make kvlayer_test run the performance
tests.

The important thing about this change is that the pytest-cov extension
registers some hooks into multiprocessing that cooperate with pytest-xdist,
but these don't play well with standalone multiprocessing-oriented tests
(and running multiprocessing under multiprocessing is bad luck anyways).

You need to have pytest in the environment (or install kvlayer[unittest])
for kvlayer_test to run unit tests.  Similarly, the standalone kvlayer tool
will use dblogger (and its -q/-v options) for basic logging setup if it is
in the environment, but does not depend on it, breaking a circular dependency
here.