Update test_overlay.py

This test is run daily on Jenkins and always fails because there are not sufficient people running the barter community yet.
Less verbose database logging.

- SQL statements are now at level NOTSET, previously DEBUG
- All logging output now contains [file_path]
- Journal mode is left unchanged for :memory: databases
Merge pull request #93 from boudewijn-tribler/bugfixes

Less verbose database logging.
Fixes incorrect wire protocol documentation.
Fixes missing _candidates.

Fixes tracker statistics reporting, would previously crash the
periodic report when attempting to use non existing _candidates
property.
Merge pull request #94 from boudewijn-tribler/bugfixes

Fixes incorrect wire protocol documentation.
Iterate over available conversions when a message is made.

Previously when a message is made the default conversion was used,
unless otherwise specified.  Now we will iterate (in reverse order)
over the available conversions and use the first (i.e. latest)
conversion that is capable of encoding the message.
Silly bug.
Changes _conversions from dictionary into a list.
Asserts no longer apply.
The method is_any_active no longer exists.
Merge pull request #97 from boudewijn-tribler/bugfixes-two

The method is_any_active no longer exists.
Cleanup for get_conversion().
Documentation and assert cleanup.
Clarified docstring.
Merge pull request #95 from boudewijn-tribler/bugfixes

Fixes missing _candidates.
Allow enabling of individual overlay tests.
Output Dispersy statistics just before shutdown.
Merge pull request #99 from boudewijn-tribler/bugfixes

Unit test tweak and Dispesy statistics output
Split get_conversion(USING) into three methods.

- avoid confusion with the property USING
- use similar schema as get_member, get_member_from_id