Instead of relying on combination of hashBlock and nIndex
values to manage tx in its lifecycle, we introduce 4
status : CONFIRMED, UNCONFIRMED, CONFLICTED, ABANDONED.

hashBlock and nIndex magic values should only be used at
serialization/deserialization for backward-compatibility.

At block disconnection, we know flag txn as UNCONFIRMED where
previously they kept their states until being override by a
block connection or abandontransaction call. This is a change
in behavior for which user may have to call abandon twice
if transaction is disconnected and not accepted back in the mempool.

We assert status transitioning right in AddToWallet. Doing so
flagged a misbehavior in ComputeTimeSmart unit test where same
tx is confirmed twice in different block. To avoid inconsistencies
we unconfirmed tx before new connection in different block. We
also remove a cs_main lock in test, as AddToWallet and its
callees don't rely on locked chain.

https://github.com/bitcoin/bitcoin/pull/16624/commits/a31be09bfd77eed497a8e251d31358e16e2f2eb1

Partial backport of Core PR16624