init: allow shutdown during 'Activating best chain...'
Two-line patch to make it possible to shut down bitcoind cleanly during
the initial ActivateBestChain.
Fixes #6459 (among other complaints).
To reproduce:
- shutdown bitcoind
- copy chainstate
- start bitcoind
- let the chain sync a bit
- shutdown bitcoind
- copy back old chainstate
- start bitcoind
- bitcoind will catch up with all blocks during Init()
(the boost::this_thread::interruption_point / ShutdownRequested()
dance is ugly, this should be refactored all over bitcoind at some point
when moving from boost::threads to c++11 threads, but it works...)