This should solve #2675 and #1981
By preloading the chunks in memory before sending them, we don't keep the
file open and therefore we let other program open the file for writing.
If the file is modified between two chunks, we detect that and abort anyway
* Use a shared pointer to Account everywhere to ensure
the instance stays alive long enough for a sync to terminate
* Folder is now tied to an AccountState
* SyncEngine and OwncloudPropagator tie to an Account and use that
for all jobs they run
Issue: Since the setup wizard currently always replaces the
account, it will always wipe all folder definitions, even when
the actual changes to the account were minor.
