Fix double PID registration race

Fix double PID registration race

Introduce a second hash table indexed by application socket which have
the exact same content as the hash table indexed by PID.

On unregister, we now use a direct lookup per socket instead of using
the key map between sock and PID. This prevents the PID-sock lookup race
when the unregister happens just after the replace and before the
close(fd).

We also use an add_replace call on application registration for the PID
hash table and kept the add_unique for the socket hash table.

(closes #7)

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>