Fix: assigning to new owner and revoking old assignees permissions upon lead_owner change

[pu-raihan]

This PR is a fix to this issue #417
Previously the share permissions were revoked from the assignees each time the lead and deal get evaluated.
this PR checks if the owner of doc has changed and then revokes the perms.

Code Changed

if not self.is_new():
   		curr_owner = frappe.db.get_value(self.doctype,self.name,"lead_owner")
   		if self.lead_owner and self.lead_owner != curr_owner:
   			self.share_with_agent(self.lead_owner)
   			self.assign_agent(self.lead_owner)