Don't reload event by UID if already in memory.

[dcaliste]

Don't reload an existing incidence from DB.
Either the calendar is already in sync with
the calendar or the database has been externally
modified and in that case, the calendar has been
emptied.

Testing the parent incidence in case of recurring
incidence should be enough to guarantee the
existence of all exceptions since the load
methods ensure loading of whole series.

@pvuorela, tests are passing and this PR follow the same spirit as the one introducing the caching for ranges, meaning that there is no point in reloading if we assume that the calendar is in sync with the DB. It is a minor optimisation that should not cost too much in term of maintainablity. It may not be so minor though for search cases where there could be large quantity of events to load by UIDs.

[pvuorela]

Somewhere on the borderline here, but since the range is indeed already tracking the loaded range and omitting those. And since the check should indeed guarantee the event is loaded already, let's just have this.