Wallet create causing crash when confirming blocks

[wezrule]

This bug was found by @PlasmaPower and nicely documented here.

I think this can be resolved by checking that the handle is valid inside wallet_store::begin. There doesn't seem to be a standard way to test that so I'm just checking that the mdb_dbi_flags doesn't return an error (that function calls TXN_DBI_EXIST which checks that the transaction and dbi arguments are correct and returns EINVAL if they aren't), a few others did that too but this one seemed to make the most sense.

The new slow test added seems to more often than not hit the bug on a single iteration, but doing 5 in a loop just to increase the probability.

Couple unrelated changes I made: Added [[maybe_unused] in receive_confirmed & removed an unnecessary std::move in wallets::queue_wallet_action

[PlasmaPower]

Given the latest commit, I should note that I'm not sure I found the only possible scenario where a wallet transaction is created and then get_wallets is called.

[wezrule]

The only other place is search_pending which was already creating the transaction after calling get_wallets (). However they both didn't account for deletes so now getting the read snaphot under the wallets mutex as well to make sure they are in a valid state.