Move GenerateBitcoins() call out of net.cpp's StartNode()

The internal miner is closely bound to the wallet engine,
not the blockchain engine.

This small cleanup was cherry-picked from #2154 because it is useful beyond the obvious merits:  it gets closer to my goal of fork()'ing away the blockchain engine.

Note that this commit places GenerateBitcoins() after RPC server start, unlike #2154 which employs a stub thread.