…, unless overridden

fa4db86 test: Activate all regtest softforks at height 1, unless overridden (MarcoFalke)
faad1e5 Introduce -testactivationheight=name@height setting (MarcoFalke)
fadb2ef test: Add extra_args argument to TestChain100Setup constructor (MarcoFalke)
faa4698 test: Remove version argument from build_next_block in p2p_segwit test (MarcoFalke)
fa086ef test: Remove unused ~TestChain100Setup (MarcoFalke)

Pull request description:

  All softforks that are active at the tip of mainnet, should also be active from genesis in regtest. Otherwise their rules might not be enforced in user testing, thus making their testing less useful.

  To still allow tests to check pre-softfork rules, a runtime argument can change the activation height.

ACKs for top commit:
  laanwj:
    Code review ACK fa4db86
  theStack:
    re-ACK fa4db86

Tree-SHA512: 6397d46ff56ebc48c007a4cda633904d6ac085bc76b4ecf83097c546c7eec93ac0c44b88083b2611b9091c8d1fb8ee1e314065de078ef15e922c015de7ade8bf

…tiveAtTip

The flag fDIP0001ActiveAtTip shows a status of activation of DIP0001

This flag is used currently only in net.cpp for setup block buffer size.
Assume the bigger by default so far as DIP0001 is activated years ago