Add additional hooks for subclasses

[jamadden]

Fixes #22

I see nti.site doing something like this in its subclass:

class _RunJobInSite(TransactionLoop):

    def setUp(self):
        db = component.getUtility(IDatabase)
        self.conn = db.open()
        self.tearDown = self.conn.close

That means the object can't be used from multiple threads at the same time, but it's not used in that way anyway.

Also some minor refactoring with an eye towards speed and simplicity.

We get between 5 and 10% faster, even accounting for all the overhead of raising exceptions:

Benchmark	master	this branch
trivial commit	14.9 us	13.2 us: 1.12x faster (-11%)
max retries	16.0 us	15.1 us: 1.06x faster (-5%)

[jamadden]

I need to release this project in order to make the necessary changes to nti.site. But I'd really like to see zopefoundation/transaction#84 merged and released before releasing this so that we can fix #17 and remove one of the XXX comments that explicit transactions are designed to help with in the first place.

[jzuech3]

I'd advocate we release this project as-is to get as much alpha burn in time as possible for these changes, coming back later for the transaction changes.

[jamadden]

That's a fair point. The aborthook thing is pretty minor, and I think I've thought of a way to handle the XXX thing in a safe-enough way even without the transaction update; it's mostly a hypothetical issue anyway.