2 GB pruned node support

[Sjors]

As of #59 on AWS the node is pruned once to block 504500 and then never again.

Although c-lightning works reasonably well with pruned nodes, they'r not officially supported yet (ElementsProject/lightning#1297 ). At the moment there are occasional crashes when the node tries to lookup a block that's pruned away, see e.g. ElementsProject/lightning#1502.

Once official support is in place, we can essentially revert #59:

• set prune=2000 for the bitcoind service
• replace pruneblockchain 504500 with the current block height
• get rid of the magnetic sc1 volume (or keep a really small one)