Prevent possible pauseStaking and resumeStaking repetitive calls

[TheDZhon]

Now it's possible to call one of the pauseStaking or resumeStaking methods more than once in a row.
Even though it doesn't pose any risks or inconsistencies beyond emitting extra events, the better approach would be completely preventing repetitive calls. The latter approach is used everywhere with other 'pausable' entities across the codebase.