Some sync refactoring
Some sync refactoring

- Use only one dict for storing all temporary state of a storage
- Rename functions to be internal

untitaker/vdirsyncer,hobarrera/vdirsyncer,mathstuf/vdirsyncer,untitaker/vdirsyncer,hobarrera/vdirsyncer,tribut/vdirsyncer,untitaker/vdirsyncer,credativUK/vdirsyncer,mathstuf/vdirsyncer,credativUK/vdirsyncer,tribut/vdirsyncer