This commit:
1) Makes gossip's identity package use the new method
   Expiration() that was added to the MessageCryptoService
   in the previous commit.
   Now, when an identity is added to the identity mapper -
   a timer is set and the identity is automatically purged
   after the timer expires.
   The timer is set to the time left until the identity
   is expired.
2) Adds some tests to the identity package, and makes its
   code-coverage reach 100%.
3) Moves the logic that periodically purges un-used identities
   from the gossip package to the identity package, to improve
   code cohesion.

Change-Id: I684197ff0f20031e80e9e9341042a5593eaca07a
Signed-off-by: yacovm <yacovm@il.ibm.com>