prioritisetransaction RPC: Restore compatibility with existing implementations by using satoshis for fee offset rather than BTC

#4531 was merged changing the unit from satoshis to RPC on the premise of prioritisetransaction being a newly introduced RPC, but in fact it has been around and in use out-of-tree since v0.6.1. I don't think we should bring it into 0.10 incompatibly.
params[0].get_str() is passed to PrioritiseTransaction() without filter. Then this is logged into the logfile without filter. This can be used to crash a logfile parser application, to embed malicius javascript, to add confusing log messages, etc. 
params[0] should not be passed, since hash already can be converted into an hex string for logging.