Introduce a new method intended to replace getCoinbaseRawTx(), which
provides a struct with everything clients need to construct a coinbase.
This is safer than providing a raw dummy coinbase that clients then have
to manipulate.
The CoinbaseTx data is populated during the dummy transaction generation
and stored in struct CBlockTemplate.
Expand the interface_ipc.py functional test to document its usage
and ensure equivalence.
This frees up the name getCoinbaseTx() for the next commit.
Changing a function name does not impact IPC clients, as they only
consider the function signature and sequence number.
