public access to connection pool

Sometime methods inside client (client.LookupBlock as example) select not the best connection and we don't have possibility to override method without public access to connection pool. That MR maybe not the best solution, but it help do something like:

    client, _, err := c.Pool().BestMasterchainClient(ctx)
    if err != nil {
        return ton.BlockIDExt{}, tlb.BlockInfo{}, err
    }

tonkeeper / tongo

public access to connection pool #321