Use early udev synchronisation and update of dev nodes for clustered mirrors.

Use early udev synchronisation and update of dev nodes for clustered mirrors.

When using clustered mirrors, we need device nodes to be created during
processing of device tree, not at its end like we normally do (we need to
access the nodes in cmirror prematurely). Therefore we use a new flag called
"immediate_dev_node" stored in deptree's load_properties struct to instruct the
device tree processing code to immediately synchronize with udev and flush all
stacked node operations so the nodes are prepared for use.

For now, the immediate_dev_node is used for clustered mirrors during
processing the dm_tree_preload_children code only. We can add more later if
needed.