Add block pruning functionality

This adds a -prune=N option to bitcoind, which if set to N>0 will enable block
file pruning. When pruning is enabled, block and undo files will be deleted to
try to keep total space used by those files to below the prune target (N, in
MB) specified by the user, subject to some constraints:

- The last 288 blocks on the main chain are always kept (MIN_BLOCKS_TO_KEEP),
- N must be at least 550MB (chosen as a value for the target that could
  reasonably be met, with some assumptions about block sizes, orphan rates,
  etc; see comment in main.h),
- No blocks are pruned until chainActive is at least 100,000 blocks long (on
  mainnet; defined separately for mainnet, testnet, and regtest in chainparams
  as nPruneAfterHeight).

This unsets NODE_NETWORK if pruning is enabled.

Also included is an RPC test for pruning (pruning.py).

Thanks to @rdponticelli for earlier work on this feature; this is based in
part off that work.

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