This PR makes a number of improvements to the InvalidateBlock (`invalidateblock` RPC) and RewindBlockIndex functions, primarily around breaking up their long-term cs_main holding. In addition:
* They're made safely interruptible (`bitcoind` can be shutdown, and no progress in either will be lost, though if incomplete, `invalidateblock` won't continue after restart and will need to be called again)
* The validation queue is prevented from overflowing (meaning `invalidateblock` on a very old block will not drive bitcoind OOM) (see #14289).
* `invalidateblock` won't bother to move transactions back into the mempool after 10 blocks (optimization).
Partial Backport of Bitcoin Core PR15402 (commit 9d6dcc5):
https://github.com/bitcoin/bitcoin/pull/15402/commits/9d6dcc52c6cb0cdcda220fddccaabb0ffd40068d
Abstract EraseBlockData out of RewindBlockIndex
Note that the former 'else' branch in RewindBlockIndex is now
dealt with more naturally inside the EraseBlockData call (by
checking whether the parent needs to be re-added as candidate
after deleting a child).