The Credit and Debits structures are simple wrappers around an
embedded *txstore.TxRecord, as well as an output index in the case of
Credit. This means that a Credit is at most two words, while a Debits
struct is just one. To avoid the unnecessary garbage of creating
Credit and Debits structures on the heap (where the underlying
TxRecord likely already is), simply pass around everywhere as
non-pointer types, and modify the receivers for all Credit and Debits
methods to non-pointer receivers since none of them ever modify the
value.
If a JSON array result was successfully calculated, but the
slice/array is empty, the result must be marshaled as '[]' rather than
the JSON null value. To do this in go, the RPC handlers should never
return nil slices for non-error returns, but return a non-nil slice
header with 0 length.
For example, an empty listtransactions result should be returned as
[]btcjson.ListTransactionsResult{}, rather than nil.
The gettransaction handler was attempting to lookup the "sent-to"
address of an outgoing transaction from the transaction store (as a
wallet credit). This is the incorrect address when sending to an
address controlled by another wallet, and panics when there are no
credits (for example, sending to another wallet without any change
address). Instead, use the first non-change output address is used as
the address of the "send" result.
This fixes the panic reported when debugging issue #91.
While here, fix the category strings used for wallet credits to
support immature and generate (the categories for coinbase outputs).
