Enable PreViewChange Strategy

A PreViewChange strategy was implemented in the scope of INDY-1687 to speed-up and improve stability of the view change. It's currently disabled by default.

Acceptance criteria

• Make sure that it doesn't have any side-affects
• Do more load testing (with regular view changes) with strategy enabled:
  • do 10 NYMs per sec
  • check that most of view changes are quite fast
  • check that view changes don't lead to broken ledgers (all ledger are in sync eventually)
  • enable the strategy by default