Add configuration for networks without blocks

One of the partners using the Hathor Network blockchain does not use blocks. In these cases, the `tx-mining-service` keeps throwing errors related to the lack of blocks, most notably:

https://github.com/HathorNetwork/tx-mining-service/blob/047e3027fba7cb92423e93fca0a212eaf70f0c49/txstratum/manager.py#L313

https://github.com/HathorNetwork/tx-mining-service/blob/047e3027fba7cb92423e93fca0a212eaf70f0c49/txstratum/manager.py#L472

These errors end up creating noise in monitoring platforms, so there should be a configuration to stop `tx-mining-service` from trying to interact with blocks.