Process events when destroying CoreNetwork

Process events when destroying CoreNetwork

During shutdown, process events in CoreNetwork, and wait for the
socket to disconnect.  This fixes the QUIT command not getting sent
to IRC networks.

Examples
[Unreal 3.2]
> Before
<-- dcircuit_dev (quasseldev@hostmask.IP) has quit (Input/output error)
> After
<-- dcircuit_dev (quasseldev@hostmask.IP) has quit (Quit: My Message!)
[Freenode]
> Before
<-- dcircuit_dev (~quasselde@hostmask) has quit (Remote host closed the connection)
> After
<-- dcircuit_dev (~quasselde@hostmask) has quit (Quit: My Message!)

Where "My Message!" is specified in Configure Quassel -> IRC
-> Identities -> Advanced -> Quit Reason

Note: Freenode hides quit messages from clients that disconnect soon
after connecting.  Stay connected ~10 minutes before testing QUIT.

(cherry picked from commit 59ed0127591f946a68a6ee7f30b23deb37d26821)