test p2p mirrornet: wait for block instead of assert if it exists
Because sometimes WAL has latency with dumping blocks to database, it is better to wait for a block instead of expecting it immediately after entering to live sync
- another possible solution is dumping to irreversible blocks (instead of reversible) all the data cached and not already dumped during P2P state. This way WAL won't be used in the test
- The best solution, find out why WAL sometimes stuck for a while and fix the root cause