Commit 763d714f introduced cron job locking for databases which had
modules with states set to 'to x', however if an
installation/uninstallation/upgrade fails, the state will stay at 'to
x', and it may stay in that state for an indefinite amount of time,
meaning that cron jobs could stay locked forever.

This commit fixes this in part by adding a cleanup function to loading.py that
will be executed whenever load_modules fails, the function will change
every 'to x' module to their original state, effectively unlocking the
execution of cron jobs.

This however only works to prevent "zombie" transient states for
brand new databases, however for existing databases which already
contain some modules in a zombie state it won't do anything unless
a module is installed/uninstalled/upgraded, which may never happen.

This is where the second part comes in (ir_cron.py), when failing to
execute crons, we check if the failure was due to bad module state
and if an arbitrary amount of time (5 hours as of this commit) has passed
since the last time it was supposed to be executed, if it is the case, it means
that the cron execution failed around 5 * 60 times (1 failure per minute for 5h)
in which case we assume that the crons are stuck because the db
has zombie states and we force a call to reset_module_states.