deadlock when disconnecting dynamic session

[shipa988]

this kind of writing in channel makes deadlock possibility!

quickfix/session.go

Line 742 in 7945e4b

 s.stateTimer = internal.NewEventTimer(func() { s.sessionEvent <- internal.NeedHeartbeat }) 

Possible deadlock occurs in dynamic session closing because line

quickfix/session.go

Line 768 in 7945e4b

 case evt := <-s.sessionEvent: 

is the only reader from channel s.sessionEvent, and after ending of loop

quickfix/session.go

Line 752 in 7945e4b

 for !s.Stopped() { 

no one can read from channel s.sessionEvent

PR #523

Thanks!