Add HACKING file to document coding style, etc.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
Update FIXME comments to match HACKING guidelines

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
core: Fix bug for duplicate notifications.

Once a client is terminated, we need to clear out the notfications hashmap.
test: Cover case of terminate client removing notifications
HACKING: fix a typo.