cells: remove System time dependency in unit-test

The CellNameServiceRegistryTests class provides unit-tests for the
CellNameServiceReigstry.  One of these tests verifies that expired
well-known cells registration is removed successfully, leaving the
existing cells intact.

This patch refacts CellNameServiceRegistry slighly to allow a
subclass to adjust its concept of time.  This is used to provide a
reliable simulated flow of time in the unit-tests, removing the
need for a sleep statement.

Target: master
Patch: https://rb.dcache.org/r/7090/
Acked-by: Tigran Mkrtchyan
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6