Default fPayAtLeastCustomFee to false

This allows for much finer control of the transaction fees per kilobyte
as it prevent small transactions using a fee that is more appropriate
for one that is of a kilobyte.

This also allows controlling the fee per kilobyte over rpc such that:

bitcoin-cli settxfee `bitcoin-cli estimatefee 2`

would make sense, while currently it grossly fails often by a factor of x3

src/wallet/wallet.cpp

@@ -41,7 +41,7 @@ CAmount maxTxFee = DEFAULT_TRANSACTION_MAXFEE;
 unsigned int nTxConfirmTarget = DEFAULT_TX_CONFIRM_TARGET;
 bool bSpendZeroConfChange = DEFAULT_SPEND_ZEROCONF_CHANGE;
 bool fSendFreeTransactions = DEFAULT_SEND_FREE_TRANSACTIONS;
-bool fPayAtLeastCustomFee = true;
+bool fPayAtLeastCustomFee = false;
 
 /**
  * Fees smaller than this (in satoshi) are considered zero fee (for transaction creation)