I noticed the log message was always printing "CreateNewBlock(): total size: 81 ...",
which is a bit silly.
This moves the pblock population back to where it belongs, right after the
`entries` are determined. To make this work, the coinbase tx is inserted
into pblock directly which is also what we want (and this goes back to what
Core does).
Note that this leaves `pblocktemplate->entries[0]` with a blank transaction
ref, but this is not important for anything, since nothing uses it.