Alerts might be dropped

[simonpasquier]

While investigating a flaky test, I found out that the iterator returned by (*github.com/prometheus/alertmanager/provider/mem.Alerts).Subscribe() might miss alerts that would be added in-between. I've also modified (*github.com/prometheus/alertmanager/storemem.Alerts).List() to return a slice rather than a closed buffered channel which I feel is more idiomatic.

If you run TestAlertsSubscribe several times without the fix, it should exhibit the issue:

$ go test -run TestAlertsSubscribe -count 100 -race  ./provider/mem/.
--- FAIL: TestAlertsSubscribe (5.03s)
    mem_test.go:238: Unexpected number of alerts for iterator 99, got: 2, expected: 3
--- FAIL: TestAlertsSubscribe (5.04s)
    mem_test.go:238: Unexpected number of alerts for iterator 99, got: 2, expected: 3