[tm-bench] rethink implementation

[melekes]

Problem

tm-bench has an issue in the sense that it may receive a block with txs after it's finished (because of the network) when the block was in fact created within this T sec, hence should be included in the results.

Solutions

1. Wait for a final non-empty block from TM

To calculate stats correctly, we can wait for the last non-empty block from TM (we just need to check block creation time to make sure it was created within period T sec).

2. Fetch the blocks TM processed within the measurement period (T sec)

Or, we can completely abandon subscribe and fetch blocks using RPC again until we hit an empty block.

I think we should go with whatever is an easier option.