ENH: allow per-operation connection for thread-safety.

The main use case of this library is integration with
cachecontrol. When a requests.Session is used in multiple threads, the
underlying adapter created by cachecontrol will also be used from
multiple threads, implicitely re-using the same sqlite connection. This
does not work.

Instead, we add a mode to SqliteCache to use a new connection for each
get/set/delete.
MAINT: fix tox configuration to actually run tests.

[skip ci]
BUG: use separate connections in threaded tests.
Merge pull request #6 from cournape/fix_multi_threading

ENH: allow per-operation connection for thread-safety.