Switch over to new btcjson.GetRawMempoolResult.

Rather than using a type specifically in btcd for the getrawmempool, this
commit, along with a recent commit to btcjson, changes the code over to
use the type from btcjson.  This is more consistent with other RPC results
and provides a few extra benefits such as the ability for btcjson to
automatically unmarshal the results into a concrete type with proper field
types as opposed to a generic interface.