<!-- Describe the issue -->

When our chainstate activation sequence fails and we're running the GUI, we ask the user via a question box whether or not they'd like to re-run the sequence with `fReindex = true`. Fun fact: despite the fact that this sequence is wrapped in a while loop, we only run it twice at most.

https://github.com/bitcoin/bitcoin/blob/7ecb309c4ed1bb9ae6b9a12cbb4f083328edc125/src/init.cpp#L1539-L1556

If they answer yes, the sequence will always fail, because a chainstate will have already been initialized in the original run and `InitializeChainstate` will throw a `std::logic_error`:

https://github.com/bitcoin/bitcoin/blob/7ecb309c4ed1bb9ae6b9a12cbb4f083328edc125/src/validation.cpp#L4573-L4594

This always reproduces. To reproduce:
```
gdb ./src/qt/bitcoin-qt
(gdb) b InitializeChainstate
(gdb) # add breakpoint for the "if (!fLoaded && !ShutdownRequested()) {" line
(gdb) r  # should stop at the breakpoint for InitializeChainstate
(gdb) c  # to advance to the breakpoint for the "if (!fLoaded && !ShutdownRequested()) {" line
(gdb) set fLoaded=false
(gdb) c  # should stop at the breakpoint for InitializeChainstate
(gdb) n  # step through InitializeChainstate to see that it throws
```

We expect the re-run of the chainstate activation sequence to not always fail.

* Provides an interface for creating and interacting with one or two
 * chainstates: an IBD chainstate generated by downloading blocks, and
 * an optional snapshot chainstate loaded from a UTXO snapshot. Managed
 * chainstates can be maintained at different heights simultaneously.
 *
 * This class provides abstractions that allow the retrieval of the current
 * most-work chainstate ("Active") as well as chainstates which may be in
 * background use to validate UTXO snapshots.
 *
 * Definitions:
 *
 * *IBD chainstate*: a chainstate whose current state has been "fully"
 *   validated by the initial block download process.
 *
 * *Snapshot chainstate*: a chainstate populated by loading in an
 *    assumeutxo UTXO snapshot.
 *
 * *Active chainstate*: the chainstate containing the current most-work
 *    chain. Consulted by most parts of the system (net_processing,
 *    wallet) as a reflection of the current chain and UTXO set.
 *    This may either be an IBD chainstate or a snapshot chainstate.
 *
 * *Background IBD chainstate*: an IBD chainstate for which the
 *    IBD process is happening in the background while use of the
 *    active (snapshot) chainstate allows the rest of the system to function.
 */
class ChainstateManager
{
private: