Search-Status Endpoint doesn't show running

## Gentics Mesh Version, operating system, or hardware.
- v0.22.1

## Problem

I noticed that the response from the `/search/status`-endpoint has a field `indexSyncRunning`.
However, that field is always set to false and never changes as far as I can tell.

Seems like there's still a TODO there: https://github.com/gentics/mesh/blob/master/elasticsearch/src/main/java/com/gentics/mesh/search/index/AdminIndexHandler.java#L58

## Reproducer

Start the index sync via `POST` `/api/v1/search/sync`. Returns the message `The index sync process was started.`.

## Expected behaviour and actual behaviour

That the `indexSyncRunning` reflects if the sync is actually running.

Start in elasticsearch/src/main/java/com/gentics/mesh/search/index/AdminIndexHandler.java at the TODO mentioned in the issue. Run POST /api/v1/search/sync, then query /search/status while synchronization is in progress and confirm that indexSyncRunning reports the actual state and changes when the process finishes.

Written by the indexing model from the issue text.