Rationalise LHCb::Event namespace usage

• drop LHCb::v2::Event (as it won't "nest" properly)
• event model related classes go into LHCb::Event
• if there is a change, and the 'old' class needs to stick around, then put the new class in a 'versioned' namespace, e.g. namespace vN where N is one larger than the previous version number (as was eg. already done for v2::Track)
• adapt code to the above rule
• rename LHCb::v2::Tracks into LHCb::Event::v3::Tracks as v1 and v2 tracks were already known
• remove v2 from namespace of stuff that does not need to have a version

must be applied in conjunction with Rec!2510 (merged) and Phys!977 (merged)