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