Fix issues related to the 2015-04-02 14:32 CEST deadlock

[stapelberg]

Symptom: clients disconnected after a while, /debug/pprof/goroutines reveals that > 70 goroutines are stuck on acquiring the applyMu mutex.

From the log on alp:

I0402 12:32:55.480121       1 api.go:97] Proxying request ("/robustirc/v1/0x13ced24bfa07d924/message") to leader "ridcully.robustirc.net:60667"
I0402 12:32:55.494351       1 api.go:97] Proxying request ("/robustirc/v1/0x13cad43f066bfec3/message") to leader "ridcully.robustirc.net:60667"
I0402 12:32:55.508024       1 robustirc.go:358] Apply(msg.Type=irc_from_client)
I0402 12:32:55.510380       1 server.go:1775] http: panic serving 172.17.42.1:40364: Assumption violated: current time 1427977975510311504 is older than the timestamp of the last processed message (1427977975515627970)
goroutine 3075187 [running]:
net/http.func·011()
        /home/michael/go/src/net/http/server.go:1130 +0xbb
github.com/robustirc/robustirc/ircserver.(*IRCServer).NewRobustMessage(0xc208354000, 0x2, 0x13ced089c4d38cbf, 0x0, 0xc221be8d80, 0xe, 0x0)
        /home/michael/gocode/src/github.com/robustirc/robustirc/ircserver/ircserver.go:239 +0x247
main.handlePostMessage(0x7fe9102a19c8, 0xc220823d60, 0xc2194c2dd0, 0xc211856f60, 0x1, 0x1)
        /home/michael/gocode/src/github.com/robustirc/robustirc/api.go:184 +0x717
github.com/julienschmidt/httprouter.(*Router).ServeHTTP(0xc21ab040c0, 0x7fe9102a19c8, 0xc220823d60, 0xc2194c2dd0)
        /home/michael/gocode/src/github.com/julienschmidt/httprouter/router.go:293 +0x18e
net/http.(*ServeMux).ServeHTTP(0xc20800ca80, 0x7fe9102a19c8, 0xc220823d60, 0xc2194c2dd0)
        /home/michael/go/src/net/http/server.go:1541 +0x17d
net/http.serverHandler.ServeHTTP(0xc21bbc1500, 0x7fe9102a19c8, 0xc220823d60, 0xc2194c2dd0)
        /home/michael/go/src/net/http/server.go:1703 +0x19a
net/http.(*conn).serve(0xc21f1695e0)
        /home/michael/go/src/net/http/server.go:1204 +0xb57
created by net/http.(*Server).Serve
        /home/michael/go/src/net/http/server.go:1751 +0x35e
I0402 12:32:55.613561       1 robustirc.go:358] Apply(msg.Type=irc_from_client)

There are two problems to be addressed:

1. panic() calls in HTTP handlers should lead to RobustIRC exiting. That fixes the deadlock itself.
2. Possibly we should try to avoid calling NewRobustMessage on non-leaders in the first place. Still thinking about that.