Remove tx submit check and update expected best block fetching to graphQL

[refi93]

• Remove Tx submission check because repeated transaction submits seem to make the cardano-http-bridge instance more susceptible to freezing as described here: https://github.com/Emurgo/cardano-http-bridge/issues/17
• Add check for liveness of cardano-http-bridge
• Replace check for expected best block with graphql request as cardanoexplorer seems to have migrated to it (check https://explorer.cardano.org/en/ - check requests in developer console) and the old endpoint (https://cardanoexplorer.com/api/blocks/pages) takes long to reply which may indicate that it's getting deprecated