Don't have cs_wallet locked while calling each context.wallet_load_fns. A load handler can always lock cs_wallet if needed. The lock was added in 1c7e25d to satisfy TSAN. With 44c430f most of the code requiring the lock is in CWallet::AttachChain. A comment is added to warn about wallets_mutex and cs_wallet lock ordering.
Backport of core#22868.