This was introduced in #799, only accidentally passing through tests.

The name is bad: a connection logged in via assumeIdentity() is
perfectly usable yet doesn't necessarily have login flows loaded as the
funciton implementation assumes.

Will be used in the next commit to rewrite TestOlmAccount.

In some situations Synapse takes a lot of time and sometimes even fails
to respond to a request at once; to alleviate that, most timeouts are
increased from 10 to 40 seconds, to give time for a retry. And to unify
the waiting code, Quotient::waitForFuture() is introduced that
internally uses QTest::qWaitFor() that is more lightweight than
QSignalSpy.

These APIs are practically internal; the change is meant to streamline
adding an account, assuming that both the identity and the access token
arrive to these classes at the same time (e.g. see
Connection::completeSetup()).

1. Mock connections no more use ConnectionEncryptionData::setup(), just
   initialise the object directly. There's not that much work, and it's
   entirely independent from what CED::setup() normally does.
2. Clearing the database has moved from Connection::P::loginToServer()
   to CED::setup(); the previous logic almost never worked because
   encryptionData is usually initialised asynchronously now, and even
   in the rare asynchoronous cases the clearing happened at the wrong
   moment - after the first save of the newly initialised Olm account.