Previously there was one shared configuration object for all jobs,
threads and other tasks. This patch creates separate ConfigWrite
instances for distinct jobs/threads.

All exported methods of ConfigWriter are now wrapped in calls that
obtain the ConfigLock from WConfD, read the current configuration, and
optionally write it back to WConfD.

_OpenConfig is now called at each such request (instead of just once at
the creation time of ConfigWriter). A new method _CloseConfig is added
that performs the necessary cleanup (saving the configuration, releasing
the lock).

_UpgradeConfig needs to be called every time a configuration is received
from WConfd, to fix parts that aren't persisted by the Python code. This
requires that it doesn't use any methods that acquire locks, and it must
not save the configuration at the end (unless it's called just after
creating a ConfigWriter instance in "offline" mode).

The semantics of Update changes slightly. Before it just checked its
argument existed in the configuration. Now it also checks that the
its serial number is the same as in the master configuration state, to
avoid overwriting changes in other threads. This will require fixing all
calls to Update, in particular to avoid interspersing calls to Update
and other ConfigWriter methods. In the future, we should aim to
eliminate Update completely.

All LUs now carry their own instance of ConfigWriter, with their
corresponding job ID. Other cide that uses ConfigWriter identifies with
job ID 'None' and thread ID.

Signed-off-by: Petr Pudlak <pudlak@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>