This patch adds a timer to clear 'adv_entries' after three minutes.

After some amount of time, the advertising entries cached during
the last LE scan should be considered expired and they should be
removed from the advertising cache.

It was chosen a three minutes timeout as an initial attempt. This
value might change in future.
Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>