[ADDED] First draft: sendmany RPC support

We can now run payouts via sendmany:

* No big logical changes, sendmany is called once for MP and once for AP
* Instead of paying out via sendtoaddress, store in array and use
  sendmany later
* Update all transactions with the RPC TXID returned by sendmany

Will fix #1238 once merged.
[CHANGE] Made startup messages debug level