Implement Vector Clock for Sync

[Daniel Zoller]

Currently the sync uses the timestamps of the post for detecting conflicts. This has the downside, that the time of the syncing systems must be in sync, too. Vector clocks can generate a partial ordering of events without using timestamps.

Please implement a vector clock.

Steps:

1. create a sync_clock table: username, intrahash, system_uri, counter
2. add a plugin that increases the clock vector for the updated resource by one. (attention also tag updates and other batch updates must increment the clock)
3. Implement the vector clock as output of the sync plan (xml, …)
4. Implement a mechanism to update the vector clocks of other systems / adapt the sync conflict detection to use the new vector clocks