Specify fees explicitly via JSON-RPC

[luke-jr]

Status: Deferred until handling stuck transactions is implemented

If a transaction sent requires a fee greater than (new) maxtxfee option, aborts with an error.

If started with undocumented unsupported -nosafefees option, allows JSON-RPC to set a "forcetxfee" to override the error and send anyway (with maxtxfee amount of fees only).

[jgarzik]

Closing,

1. requires nak'd commit that accepts non-standard transactions
2. "status: deferred" could be for a long time
3. this is an active area of Gavin work too, and so I think it is fair to require some discussion (on bitcoin-devel?) before proposing fee calculation or fee RPC changes. You will note that CreateNewBlock: Child-pays-for-parent / Add transaction fee later #1647 is on the list for 0.7, so hopefully that illustrates we are being fair in this area, and merging discussed items.

[luke-jr]

1. It was not NAK'd.
2. So?
3. It was not at the time it was implemented, and still isn't really. Low-level tools are no substitute for the high-level ones.