Assert that things aren't randomly null. This will make it easier
to fix than trying to hunt down obscure errors in the client caused
by nulls here.

Remove CallbackManager hacks.

Doesn't really make any sense at all, and isn't used anywhere to
my knowledge.

We now pass parser/date in properly, no need to drop these and
re-add them.

Remove "non-critical" callbacks - if callers want to ignore the
exception they can just try/catch it.

Make addCallbackType private, and remove the unused delCallbackType
method.

All callbacks are now done through getCallback, which offers
type safety (gasp!) and is potentially extensible in the future
(wow!).

This means that nothing can depend on callbacks existing, so
there are two functional changes:

 - the parser will no longer disconnect automatically on failed
   pings; users have to listen for the event and handle it
   themselves;

 - the parser will no longer attempt to automatically use alt
   nicknames or create new nicknames on connection; users have to
   listen for the event and implement the logic themselves

Both of these were really weird anyway, as adding a listener (even
for debugging purposes) disabled the built-in behaviour.

Change-Id: Ia494a8e1d61ab66c7b305d274746526f6d8e5f83
Reviewed-on: http://gerrit.dmdirc.com/4075
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager

Change-Id: I1d81392650d8193503621b2e69fbe66cd0eb736a
Reviewed-on: http://gerrit.dmdirc.com/4058
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager