Reworked send.sh, so it works properly on my Mac (killall send.sh
doesn't work, because the process name is 'bash' not 'send.sh').
So now send.sh writes a .send.pid file, and invoking it as
send.sh -STOP (as the bitcoind -walletnotify) signals that PID.
Add a function `WaitBlocks` to wait for blocks to propagate to all three
nodes, and use this instead of waiting a fixed time of one second.
Fixes#3445.
qa/rpc-tests/wallet.sh runs a three-node -regtest network,
generates a fresh blockchain, and then exercises basic wallet
sending/receiving functionality using command-line RPC.
