Merge #17537: wallet: Cleanup and move opportunistic and superfluous TopUp()s

6e77a7b65cda1b46ce42f0c99ca91562255aeb28 keypool: Add comment about TopUp and when to use it (Andrew Chow)
ea50e34b287e0da0806c1116bb55ade730e8ff6c keypool: Move opportunistic TopUps from LegacyScriptPubKeyMan to CWallet and ReserveDestination (Andrew Chow)
bb2c8ce23c9d7ba8d0e5538243e07218443c85b4 keypool: Remove  superfluous topup from CWallet::GetNewChangeDestination (Andrew Chow)

Pull request description:

  * The `TopUp()` in `CWallet::GetNewChangeDestination` is unnecessary as currently m_spk_man calls TopUp further down the call stack inside LegacyScriptPubKeyMan::ReserveKeyFromKeyPool (called by LegacyScriptPubKeyMan::GetReservedDestination). This also lets us prepare for future changes with multiple ScriptPubKeyMans in the wallet.
  * An opportunistic `TopUp()` is moved from `LegacyScriptPubKeyMan::GetNewDestination` to `CWallet::GetNewDestination`.
  * Another opportunistic `TopUp()` is moved from `LegacyScriptPubKeyMan::ReserveKeyFromKeyPool`

  Moving opportunistic TopUps ensures that ScriptPubKeyMans will always be topped up before requesting Destinations from them as we cannot  always rely on future ScriptPubKeyMan implementaions topping up internally.

  See also: https://github.com/bitcoin/bitcoin/pull/17373#discussion_r348598174

ACKs for top commit:
  instagibbs:
    utACK https://github.com/bitcoin/bitcoin/pull/17537/commits/6e77a7b65cda1b46ce42f0c99ca91562255aeb28 only change is slight elaboration on comment
  ryanofsky:
    Code review ACK 6e77a7b65cda1b46ce42f0c99ca91562255aeb28. Only the comment changed since my previous review.

Tree-SHA512: bdfc8d303842c3fb7c3d40af7abfa6d9dac4ef71a24922bb92229674ee89bfe3113ebb46d3903ac48ef99f0a7d6eaac33282495844f2b31f91b8df55084c421f

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