The min-difficulty blocks are incompatible with difficulty averaging.
Network difficulty is also now defined as the difficulty the network is
currently working to solve, rather than the last non-min-difficulty block
difficulty.
This was unintentionally committed, and caused Equihash verification of blocks
without parents to be skipped. This only affects the genesis block on the test
network, but also causes the "time verifyequihash" benchmark to incorrectly
appear instantaneous.
The genesis blocks and miner tests have been regenerated, because changing the
block header serialisation format changes the block hash, and thus validity.
The Equihash solutions have been removed from the bloom test inputs for
simplicity (block validity is not checked there; only a valid serialisation is
necessary).
When the difficulty adjustment algorithm was altered, the special testnet
min-difficulty case was maintained, but the difficulty adjustment for the
following block then adjusted from min-difficulty instead of from the last
non-min-difficulty block. This caused the difficulty on the testnet to sawtooth
instead of stabilising. The intended behaviour is restored here.
Changing the order of difficulty calculation operations to divide first doesn't
affect the result significantly, but ensures we never overflow the arith_uint256
during multiplication and get an artificial jump in difficulty.
The main and test networks are configured to use parameters that are currently
low-memory but usable with the basic solver; they will be increased once the
solver is optimised. The regtest network is configured to have extremely low
memory usage for speed.
Note that Bitcoin's double-hasher is used for the difficulty check. This does
not match the paper, but is simpler than changing the block header
serialization. Single hashing is kept for the EquiHash solver because there is
no requirement on execution time there, only on memory usage.
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.
