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
The server is slow to process each chunk. As a result the time between
the chunks is not neglectible. Therefore we gain a lot of time by
uploading the chunks in parallel. So the bandwitdh is used when the
server is waiting between chunks.
* 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.
