Merge #18192: Bugfix: Wallet: Safely deal with change in the address book

b5795a788639305bab86a8b3f6b75d6ce81be083 Wallet: Add warning comments and assert to CWallet::DelAddressBook (Luke Dashjr)
6d2905f57aaeb3ec3b63d31043f7673ca10003f2 Wallet: Avoid unnecessary/redundant m_address_book lookups (Luke Dashjr)
c751d886f499257627b308b11ffaa51c22db6cc0 Wallet: Avoid treating change-in-the-addressbook as non-change everywhere (Luke Dashjr)
8e64b8c84bcbd63caea06f3af087af1f0609eaf5 Wallet: New FindAddressBookEntry method to filter out change entries (and skip ->second everywhere) (Luke Dashjr)
65b6bdc2b164343ec3cc3d32a0297daff9e24fec Wallet: Add CAddressBookData::IsChange which returns true iff label has never been set (Luke Dashjr)
144b2f85da4d51bf7d72b987888ddcaf5b429eed Wallet: Require usage of new CAddressBookData::setLabel to change label (Luke Dashjr)
b86cd155f6f661052042048aa7cfc2a397afe4f7 scripted-diff: Wallet: Rename mapAddressBook to m_address_book (Luke Dashjr)

Pull request description:

  In many places, our code assumes that presence in the address book indicates a non-change key, and absence of an entry in mapAddressBook indicates change.

  This no longer holds true after #13756 (first released in 0.19) since it added a "used" DestData populated even for change addresses. Only avoid-reuse wallets should be affected by this issue.

  Thankfully, populating DestData does not write a label to the database, so we can retroactively fix this (so long as the user didn't see the change address and manually assign it a real label).

  Fixing it is accomplished by:

  * Adding a new bool to CAddressBookData to track if the label has ever been assigned, either by loading one from the database, or by assigning one at runtime.
  * `CAddressBookData::IsChange` and `CWallet::FindAddressBookEntry` are new methods to assist in excluding change from code that doesn't expect to see them.
  * For safety in merging, `CAddressBookData::name` has been made read-only (the actual data is stored in `m_label`, a new private member, and can be changed only with `setLabel` which updates the `m_change` flag), and `mapAddressBook` has been renamed to `m_address_book` (to force old code to be rebased to compile).

  A final commit also does some minor optimisation, avoiding redundant lookups in `m_address_book` when we already have a pointer to the `CAddressBookData`.

ACKs for top commit:
  ryanofsky:
    Code review ACK b5795a788639305bab86a8b3f6b75d6ce81be083. Pretty clever and nicely implemented fix!
  jonatack:
    ACK b5795a788639305bab86a8b3f6b75d6ce81be083 nice improvements -- code review, built/ran tests rebased on current master ff53433fe4ed06893d7c4 and tested manually with rpc/cli
  jnewbery:
    Good fix. utACK b5795a788.

Tree-SHA512: 40525185a0bcc1723f602243c269499ec86ecb298fecb5ef24d626bbdd5e3efece86cdb1084ad7eebf7eeaf251db4a6e056bcd25bc8457b417fcbb53d032ebf0

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