Source: Add fallback services and policies

[gdassori]

Services like Blocktrail API may provide block transactions, but a bit slowly. cause a large amount of data is returned, and not only txids.

Need some sort of "fallback services" to pick data from, if it's missing from the final artifact.
But this reduces the already low entropy of the sources, and should be done only for big queries (like blocks, and not rawtransactions).

To enable a choice, we may introduce "fullmode" and "fastmode" after #5 is done.

In "full mode" fallback services may be used as everything else, in "fast mode" they're used only when data is missing, and this introduce the need of final artifact validation.

Just public brainstorming.

[gdassori]

partially addressed by 352b792 for missing rawtxs.
use electrum as fallback.

[gdassori]

There is no evidence of more aggressive fallback policies on public services.
I'll use the bitcoin p2p, instead, to fetch full blocks.