[WIP] Do trust check on each Agent config update.

In order to mitigate problems caused by removing tombstones, do trust checks each
time we synchronize Agent config from ZK. Trust check is implemented as a transaction
of two checks - one check ensures that the version of /config/hosts/<host>/jobs
has not changed since we read it just before the check, and the second one checks if
the /config/id node is present.