Make allowances for session timeout resetting UNION default behaviour

[ADBond]

Closes #33.

Introduces function ClickhouseAPI.set_union_default_mode(), which in addition to be called on object initialisation can be called independently. This makes it easier for users running interactively, or otherwise in some script where there may be a long period of non-interaction with the db, to re-set the setting necessary for clustering (and possibly other functions).

For now this seems preferable to automatically setting this e.g. every time we execute SQL. But if this proves to be irritating then we can always trigger this function when certain conditions are met.

Additionally to avoid this potentially coming up in tests we change from having DatabaseAPI fixtures to making them factory fixtures, so that the apis only get instantiated within tests rather than for the lifetime of the fixture.