dbus: Add D-Bus methods to flush the BSS cache

dbus: Add D-Bus methods to flush the BSS cache

Add an "Interface.FlushBSS" method to the new D-Bus API and a "flush"
method to the old API. Both take an age parameter that is a threshold
(in seconds) for selecting entries to evict. Setting this parameter
to zero flushes all entries from the cache.

This mechanism is useful for a connection manager to clear state at
startup and on resume (where the age parameter may be used to hold
onto recent/valid data).