Add configuration for networks without blocks

[tuliomir]

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:

tx-mining-service/txstratum/manager.py

Line 313 in 047e302

self.log.exception("Error updating block template")

tx-mining-service/txstratum/manager.py

Line 472 in 047e302

"Cannot generate MinerBlockJob because block_template is empty"

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.