Fix memleak in TorController #7610

It looks like, TorController::disconnected_cb(TorControlConnection& conn) gets called multiple times which results in multiple event_new().

Needs testing!

2016-02-26 19:29:42 setKeyPool.size() = 101
2016-02-26 19:29:42 mapWallet.size() = 2202
2016-02-26 19:29:42 mapAddressBook.size() = 4
2016-02-26 19:29:42 init message: Loading addresses...
2016-02-26 19:29:42 torcontrol thread start
2016-02-26 19:29:42 tor: Error connecting to Tor control socket
2016-02-26 19:29:42 tor: Not connected to Tor control port 127.0.0.1:9051, trying to reconnect
2016-02-26 19:29:43 Loaded 0 addresses from peers.dat  1732ms
2016-02-26 19:29:43 tor: Error connecting to Tor control socket
2016-02-26 19:29:43 tor: Not connected to Tor control port 127.0.0.1:9051, trying to reconnect
2016-02-26 19:29:43 init message: Loading banlist...
2016-02-26 19:29:45 tor: Error connecting to Tor control socket
2016-02-26 19:29:45 tor: Not connected to Tor control port 127.0.0.1:9051, trying to reconnect
2016-02-26 19:29:46 dnsseed thread start
2016-02-26 19:29:46 net thread start

utACK nit: event_free implies event_del, doing both is unnecessary

Why free/create every time the event is set? They're reusable. How about just event_new in the ctor, event_free in the dtor, and event_add as needed?

Edit: The naming may be confusing to those unfamiliar with libevent. event_new allocates, event_add schedules a new event, event_del un-schedules if not yet run, event_free deallocates. I suggest just making it RAII-like.

Good point @theuni!

Closing in favor of #7637

Removing label 'needs backport'