This removes the need to include rpc/blockchain.cpp in order to put
GetDifficulty under test. GetDifficulty was called in two ways:
* with a guaranteed non-null blockindex
* with no argument
Change the latter case to be provided chainActive.Tip() explicitly.
It has no business in `rpcserver.h`. Define it in the interface header
of the implementation unit `rpcblockchain` where it is defined.
Also modernize the signature to:
double GetDifficulty(const CBlockIndex* blockindex = nullptr);
(remove `extern`, replace `NULL` with `nullptr`)
