Enh: implemented transparent update

[geektophe]

This patch implements transparent update. It prevents downtimes or void results each time a new configuration is pushed.

The deleted objects state is saved when new initial Broks are sent by a scheduler. Queries get executed from this saved state until new objects are fully loaded. Once done, the old state is erased, and queries are made from latest objects state.

To enable transparent update, set the transparent_update parameter to 1 in the livestatus configuration:

define module {
    module_name        Livestatus
    module_type        livestatus
    port               50000
    ...
    transparent_update 1
    ...
}

Also:

• Fixed a bug in itersorted bound method in regenerator class that crashed livestatus when a query was made during objects reloading. If an id could not be found, return a void list rather than crashing the whole module.
• Set a null __itersorted__ method at initialization to avoid ugly object has no attribute '__itersorted__' exceptions when service starts.
• Re-indented some (very) long and difficult to read list comprehensions