Race condition on adding handler specific middlewares

[SargtLa]

I face the race condition when I attempted to add new handler with handler level middleware while other process called RunHandlers. Router has a lock for handlers, but not for middlewares.

Here is a sample of a race warning I got:
`WARNING: DATA RACE
Write at 0x00c00060e008 by goroutine 1681:
github.com/ThreeDotsLabs/watermill/message.(*Router).addHandlerLevelMiddleware()
.../github.com/ThreeDotsLabs/watermill/message/router.go:194 +0x3ad
github.com/ThreeDotsLabs/watermill/message.(*Handler).AddMiddleware()
.../github.com/ThreeDotsLabs/watermill/message/router.go:651 +0x22d

Previous read at 0x00c00060e008 by goroutine 1777:
github.com/ThreeDotsLabs/watermill/message.(*Router).RunHandlers.func1()
..../github.com/ThreeDotsLabs/watermill/message/router.go:438 +0xa4`