improves the drain-accounts script so that it funds more accounts

this was requested by the sdk teams to simplify testing

we could also actually mint to 500 accounts, but mobilecoind gets
noticeably slower as we add more accounts to monitor, so instead
we went with this pay-round-robin approach

mobilecoind/strategies/drain-accounts.py

@@ -59,10 +59,10 @@ def parse_args() -> argparse.ArgumentParser:
                         type=int,
                         default=20,
                         help="Amount less than the balance that we attempt to send")
-    parser.add_argument("--max-accounts",
+    parser.add_argument("--num-src-accounts",
                         type=int,
                         default=-1,
-                        help="Number of accounts to pull from the account keys / destination accounts keys folders")
+                        help="Number of accounts to pull from the source account keys dir (note: we always pull all the destination keys)")
     parser.add_argument("--token-id",
                         type=int,
                         default=0,
@@ -141,20 +141,25 @@ def filename_key(filename):
         for b58 in dest_b58addresses
     ]
 
-    # Go through each account and have all their friends transact to them
-    for i, (src_account, dest) in enumerate(zip(source_accounts, dest_addresses)):
-        # If we have already done max_accounts many accounts, then stop
-        if i == args.max_accounts:
-            break;
+    # Go through each destination account and pay it once from a source account,
+    # going round-robin through the source accounts (only first args.num_src_accounts source accounts)
+    n = len(dest_addresses)
+    m = len(source_accounts) if args.num_src_accounts == -1 else args.num_src_accounts
+    for i, dest in enumerate(dest_addresses):
+        src_account = source_accounts[i % n]
 
         wait_for_accounts_sync(stub, [src_account.monitor_id], 3)
         # Get starting balance
         resp = stub.GetBalance(
             mobilecoind_api_pb2.GetBalanceRequest(monitor_id=src_account.monitor_id, token_id=args.token_id))
         balance = resp.balance
-        logging.info("Starting balance for account %s : %s", i, resp)
+        logging.info("Token-id $s balance for account %s : %s", args.token_id, i, resp)
 
-        amount = balance - args.fee
+        # This source account will be chosen for this many future iterations of the loop
+        num_payments_remaining = int((n - i) / m)
+        # Divide the remaining balance evenly across this payment and the remaining future payments
+        # but also subtract the fee we have to pay
+        amount = int(balance / (num_payments_remaining + 1)) - args.fee
 
         # Create a pool of transfers to all other accounts
         logging.info("Transferring %s to %s", amount, dest)