Add support for synching daemon in Zeth CLI

[AntoineRondelet]

It would be useful to extend the zeth sync command with a flag zeth sync --loop (or zeth sync --daemon which would be equivalent to zeth sync --loop &) that would start a process that would constantly sync newly mined blocks.
Such functionalities would be particularly useful when integrating with Autonity (and the current testnet in which blocks are mined at a very fast pace).

[dtebbs]

I hit the same issue of huge numbers of blocks when synching on bakerloo for the first time. With a large --batch-size (~1000000) it syncs quickly enough, but obviously that's not suitable in all situations (for a very busy deployment I guess it could possibly crash the client).

One though I had was that it could be helpful to store some related parameters in the network config file. For example, a default batch size (so that zeth sync always finishes in a reasonable time), or some suitable polling frequency (for the zeth sync daemon).