Previously these functions would infinitely loop if sync failed;
now they have a default timeout of 60 seconds, after which an
AssertionError is raised.
sync_blocks() has also been improved and now compares the tip
hash of each node, rather than just using block count.
Zcash: Kept block count check for a couple of tests where we use it.
The p2p_nu_peer_management and rewind_index RPC tests still start from
Sprout, because they are explicitly (and only) testing network behaviour
across network upgrades.
The mempool_tx_input_limit test is removed because the flag has been
ignored since Sapling activation, and will be removed at some point in
the near future.