If the prio. will be enough after the next block don't force fees.

  If the user was really after the fastest possible confirmation times
they would be manually setting a fee. In cases where the wallet builds
a transaction with a priority that is too low to qualify as free until
the next block, go ahead without a fee.  Confirmation frequently takes
multiple blocks even when a minimum fee is provided.
Merge pull request #2129 from gmaxwell/wallet_less_frequent_fees

If the prio. will be enough after the next block don't force fees. [wallet]