Generate event ID for each adapter

We want to see simple way for debugging events, that's why we need to use sequences for each adapter type and generate `event_id`. For example:

```ruby
events = Hanami::Events.initialize(:memory_sync)
events.subscribe('user.created') { |payload| p payload }

events.broadcast('user.created', user_id: 1) # => event_id
```

## Backlog
- [x] Generate simple UUID as a event id (eid) (https://github.com/hanami/events/pull/80)
- [ ] Use a `meta` keyword in subscribe block for getting eid
- [ ] Try to use universally Unique Lexicographically-sortable time-based Identifiers as eid (https://github.com/abachman/ulid-ruby)

## Event type

I really love idea from redis streams:

> The ID is composed of two parts: a millisecond time and a sequence number. 1506871964177 is the millisecond time, and is just a Unix time with millisecond resolution. The number after the dot, 0, is the sequence number, and is used in order to distinguish entries added in the same millisecond. Both numbers are 64 bit unsigned integers. This means that we can add all the entries we want in a stream, even in the same millisecond.

That's why, let's create event id as `time.seq`