feat: Add run_in_transaction decorator.

[ArneGudermann]

This PR adds a run_in_transaction decorator. Now you can call transaction like :

@exposed
def test(self, key):
    @run_in_transaction
    def trans(key):
        e = db.Get(db.Key.from_legacy_urlsafe(key))
	assert e
	e["new"] = "123"
	return e

return trans(key)

I think it looks cleaner.
This is only an Idea What do you think of it?

[sveneberth]

I still don't understand why we should have a method for this and not have a context manager implementation like Google's Datastore client.

@exposed
	def test(self, key):
		with db.Transaction():
			e = db.Get(db.Key.from_legacy_urlsafe(key))
			assert e
			e["new"] = "123"
			return e

is so much more simple and pythonic

[ArneGudermann]

The context of the transaction is currently handled by the viur-datastore. But we can exposed it to the core. We can discuss about that in the viur meeting.

[phorward]

is so much more simple and pythonic

Why not simply...

with skel:
    if skel.fromDB(key):
        skel["count"] += 1
        skel.toDB()

[sveneberth]

Why not simply...

skel = MySkeleton(auto_save=True)
skel["count"] += 1