Fix caps support (SMACK-361, SMACK-459).

SMACK-459 splitted the SDM identities over two member variables which
are joined by getIdentities(), therefore this method must be used to
calculated the hash for entity caps.

Also recalculate the hash if an identity is added/remvoed/changed.

git-svn-id: http://svn.igniterealtime.org/svn/repos/smack/trunk@13801 b35dd754-fafc-0310-a699-88a17e54d16e