journal: when iterating through entry arrays and we hit an invalid one keep going

journal: when iterating through entry arrays and we hit an invalid one keep going

When iterating through partially synced journal files we need to be prepared
for hitting with invalid entries (specifically: non-initialized). Instead of
generated an error and giving up, let's simply try to preceed with the next one
that is valid (and debug log about this).

This reworks the logic introduced with caeab8f626e709569cc492b75eb7e119076059e7
to iteration in both directions, and tries to look for valid entries located
after the invalid one. It also extends the behaviour to both iterating through
the global entry array and per-data object entry arrays.

Fixes: #4088