Switch to etebase_fs_cache API from local implementation

- Move caching functions to ClientState from Adapter. This was needed to hold to etebase_fs_cache object for the resource duration.
- Pass ClientState to journalsfetchjob and entriesfetchjob - makes accessing account and caching functions easy
- Switch out local caching functions to client state functions in etesyncresource.cpp
- Use username_agentId as etebase cache folders to prevent conflict between different resource instances with the same username