Currently the exact reason a transaction is removed from the mempool isn't
logged. It is sometimes detectable from context, but adding the `reason` to
the validation interface logs (where it is already passed) seems like an easy
way to disambiguate.
For example, in the case of mempool expiry, the logs look like this:
```
[validationinterface.cpp:220] [TransactionRemovedFromMempool] [validation] Enqueuing TransactionRemovedFromMempool: txid=<txid>
[txmempool.cpp:1050] [RemoveUnbroadcastTx] [mempool] Removed <txid> from set of unbroadcast txns before confirmation that txn was sent out
[validationinterface.cpp:220] [operator()] [validation] TransactionRemovedFromMempool: txid=<txid>
[validation.cpp:267] [LimitSize] [mempool] Expired 1 transactions from the memory pool
```
There is no context-free way to know $txid was evicted on the basis of expiry.
This change will make that case (and probably others) clear.
This is a backport of core#26419
Depends on D16084