Split GetNextWorkRequired() into two functions to allow the difficulty calculations to
be tested without requiring a full blockchain.
Add unit tests to cover basic difficulty calculation, plus each of the min/max actual
time, and maximum difficulty target conditions.
This corrects a bug where an exception thrown reading from the database causes the cursor to
be left open, which causes an assertion error to occur when the database is deleted (around
line 938 of init.cpp).
Ross Nicoll