This commit:
1) Moves the identity mapper into the gossip package from the service
   package, since it's not needed in the service package.
2) Extends the constructor of the identity mapper to have a callback
   that is fired whenever an identity is deleted from the identity mapper.

Change-Id: I1c9eaa47c97351518848843a9c4f2e1835f82249
Signed-off-by: yacovm <yacovm@il.ibm.com>