Remove unbounded calculation of gainPerSec and write to storage instead

[SCBuergel]

The calculation of gainPerSec in the _getCumulatedRewardsIncrement function involves an unbounded for-loop. That might create hard to predict issues whenever the function is called, e.g. upon a massive NFT airdrop.

This issue can be mitigated by increasing the value of gainPerSec in storage in the onERC721Received hook and decreasing it in the reclaimErc721Tokens function and then only reading it from storage within _getCumulatedRewardsIncrement.