gom: Fetch properties when inserted for batched writes

This could allow front-ends to avoid blocking for "times n_items" that
time when launching the batched write.
gom: Don't do get/set properties in != main thread when reading

This is the 2nd part of b14e7044bf5cbfe4b6baac4b3ce66dba7dc8f458.

We now try to avoid creating, and setting GomResource items when reading
from the database. Instead, the items are created from cached results as
needed when gom_resource_group_get_index() is called.

https://bugzilla.gnome.org/show_bug.cgi?id=728301