feat: Improve connection pool robustness

[calbera]

• add how far behind (latest block height) syncing check amongst healthy clients
• use priority queue to store most up-to-date client as the highest priority
• also allow optionally choosing a sticky connection to a configured client (ties into fix(transactor): add nonce too low fix. #54)
• also allow calling multiple of the available clients in parallel (for 1 request)