Race on WebSocket handlers

[Wondertan]

Touches every WebSocket subscriber implementation.
All implementations have map of maps. Also, there is lock which defends wrapper map, but underlying maps are not backed by any lock, what causes Race Condition.

WARNING: DATA RACE
Write at 0x00c0000a64b0 by goroutine 74:
  runtime.mapdelete_fast64()
      /usr/local/Cellar/go/1.12/libexec/src/runtime/map_fast64.go:272 +0x0
  github.com/proximax-storage/go-xpx-catapult-sdk/sdk/websocket/subscribers.(*partialAddedImpl).RemoveHandlers()
      /go/pkg/mod/github.com/proximax-storage/[email protected]/sdk/websocket/subscribers/partial_added.go:65 +0x252
  github.com/proximax-storage/go-xpx-catapult-sdk/sdk/websocket/handlers.(*partialAddedHandler).Handle.func1()
      /go/pkg/mod/github.com/proximax-storage/[email protected]/sdk/websocket/handlers/partial_added.go:46 +0x13a

Previous read at 0x00c0000a64b0 by goroutine 84:
  runtime.mapiternext()
      /usr/local/Cellar/go/1.12/libexec/src/runtime/map.go:853 +0x0
  github.com/proximax-storage/go-xpx-catapult-sdk/sdk/websocket/handlers.(*partialAddedHandler).Handle()
      /go/pkg/mod/github.com/proximax-storage/[email protected]/sdk/websocket/handlers/partial_added.go:35 +0x2cd
  github.com/proximax-storage/go-xpx-catapult-sdk/sdk/websocket.(*messageRouter).RouteMessage()
      /go/pkg/mod/github.com/proximax-storage/[email protected]/sdk/websocket/router.go:43 +0x19e

Goroutine 74 (running) created at:
  github.com/proximax-storage/go-xpx-catapult-sdk/sdk/websocket/handlers.(*partialAddedHandler).Handle()
      /go/pkg/mod/github.com/proximax-storage/[email protected]/sdk/websocket/handlers/partial_added.go:37 +0x2bc
  github.com/proximax-storage/go-xpx-catapult-sdk/sdk/websocket.(*messageRouter).RouteMessage()
      /go/pkg/mod/github.com/proximax-storage/[email protected]/sdk/websocket/router.go:43 +0x19e

Goroutine 84 (running) created at:
  github.com/proximax-storage/go-xpx-catapult-sdk/sdk/websocket.(*CatapultWebsocketClientImpl).Listen()
      /go/pkg/mod/github.com/proximax-storage/[email protected]/sdk/websocket/client.go:168 +0x182