Merge Vincent's fixes to CERN (resolving MMMMALT-1050)

3 changes (plus some require refactoring) for caldav:

• Use tzname directly if possible (translating MS names): tzidFromMSTZ has some unexplained errors sometime, e.g. not able to find the TZID for events coming from TbSync on my system. While translating requires storing the map in the class, it's much simpler than trying to match the offsets & transitions ...
• Include "SEQUENCE" in VEVENTs: OX App suite requires the sequence to increase. As it's not in AS, Z-push was not pushing any, resulting in a sequence 0 on the server side. However, OX App suite also increases the sequence itself when changing the event, so after a first modification the server side sequence is 1 while Z-push tried to push none (i.e. 0). Fixed by pulling the sequence before pushing it (we were stating the event, so not too much change in cost, except the parsing)
• Remove the _collection variable: this was used to have StatMessage bypass the fetch in some cases in which we already have the data. Replaced by extracting the part of the StatMessage used for this (as FormatState) and use it.