Add a LockChain method to CWallet to know if we can lock or query
chain state safely.
At tx loading, we rely on chain to know if hashBlock of tx is still
in main chain. If not, we set its status to unconfirmed and reset
its hashBlock/nIndex.
If wallet loaded is the wallet-tool one, all wallet txn will
show up with a height of zero. It doesn't matter as status is not
used by wallet-tool.
We take lock prematurely in CWallet::LoadWallet and CWallet::Verify
to ensure that lock order is respected between cs_main an cs_wallet.
https://github.com/bitcoin/bitcoin/pull/16624/commits/40ede992d97df38282919693dfe851c975c3b1d8
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Depends on D7049
Partial backport of Core PR16624