libtest: Added library for persist-related functions in tests.

This lib covers common tasks for creating/restarting/destroying
PersistState objects.

Also refactored test to use persist_lib provided functions.

Signed-off-by: Viktor Tusa <tusa@balabit.hu>
persist-state: Refactor and extend some PersistEntry-related functions.

Added persist_state_remove_entry.

Removing an entry from persist-state sets its in_use value to zero.
At next open of persist_state, this entry will be purged.