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 436f7d7):
https://github.com/bitcoin/bitcoin/pull/15402/commits/436f7d735f1c37e77d42ff59d4cbb1bd76d5fcfb

Depends on D4683

1. Build with Clang in Debug mode:

CXX=clang++ CC=clang cmake .. -D CMAKE_CXX_FLAGS="-Werror=thread-safety-analysis" -GNinja -DCMAKE_BUILD_TYPE=Debug
ninja check

2. Verify that the compiler has not emitted a thread-safety warning.
3. Run the node: ./src/bitcoind -regtest
4. Verify that text similar to "Assertion failed: lock ... not held ..." is not printed on stderr.