message: [refactor] Move ComputeBlockVersion into VersionBitsCache
  This also changes ComputeBlockVersion to take the versionbits cache
  mutex once, rather than once for each versionbits deployment.
diff: /* Define a virtual block time, one block per 10 minutes after Nov 14 2014, 0:55:36am */
  static int32_t TestTime(int nHeight) { return 1415926536 + 600 * nHeight; }
  +static int32_t ComputeBlockVersion(const CBlockIndex* pindexPrev, const Consensus::Params& params)
  +{
  + return g_versionbitscache.ComputeBlockVersion(pindexPrev, params);
  +}
  +
  static const std::string StateName(ThresholdState state)
  {
  switch (state) {
  