Raiden on Arbitrum: Replace block numbers with timestamps

[taleldayekh]

@andrevmatos @palango to provide details.

On L2s blocks might get generated on-demand, and therefore the average block time might change quite drastically.

Some operations in the current channel lifecycle rely on giving the other channel partner to act. This has to be true on L2s as well. We therefore propose to change timeouts from blocks to seconds, so that that invariant can be kept when block times vary.