This PR consists in two fixes. The first fixes a concurrency issues with `boost::signals2`. The second fixes a wallet model destruction while it's being used.

From boost signal documentation at https://www.boost.org/doc/libs/1_72_0/doc/html/signals2/thread-safety.html:

> When a signal is invoked by calling signal::operator(), the invocation first acquires a lock on the signal's mutex. Then it obtains a handle to the signal's slot list and combiner. Next it releases the signal's mutex, before invoking the combiner to iterate through the slot list.

This means that `UnregisterValidationInterface` doesn't prevent more calls to that interface. The fix consists in capturing the `shared_ptr<CValidationInterface>` in each internal slot.

The GUI bug is fixed by using a `Qt::QueuedConnection` in the `WalletModel::unload` connection.

ryanofsky:
  Code review ACK 41b0baf43c243b64b394e774e336475a489cca2b. Only change is moving assert as suggested
hebasto:
  ACK 41b0baf43c243b64b394e774e336475a489cca2b, tested on Linux Mint 19.3.