#4387 Refactor the usage of tags (now called events) to reflect the
new ABCI events semantics:

- Move `x/{module}/tags/tags.go` => `x/{module}/types/events.go`
- Update `docs/specs`
- Refactor tags in favor of new `Event(s)` type(s)
- Update `Context` to use new `EventManager`
- (Begin|End)Blocker no longer return tags, but rather uses new `EventManager`
- Message handlers no longer return tags, but rather uses new `EventManager`

Any component (e.g. BeginBlocker, message handler, etc...) wishing to emit an event must do so
through `ctx.EventManger().EmitEvent(s)`.

To reset or wipe emitted events: `ctx = ctx.WithEventManager(sdk.NewEventManager())`
To get all emitted events: `events := ctx.EventManager().Events()`