[wallet] [refactor] Simplify getbalance implementation #9614

This is just code cleanup, no behavior is changing. It deduplicates getbalance code and makes it comprehsible so it is not doing bizarre things like subtracting negative fees (https://github.com/bitcoin/bitcoin/pull/9167).

For each wallet transaction, the balance is updated as follows:

+        CAmount debit = wtx.GetDebit(filter);
+        const bool outgoing = debit > 0;
+        for (const CTxOut& out : wtx.tx->vout) {
+            if (outgoing && IsChange(out)) {
+                debit -= out.nValue;
+            } else if (IsMine(out) & filter && depth >= minDepth && (!account || *account == GetAccountName(out.scriptPubKey))) {
+                balance += out.nValue;
+            }
+        }
+
+        if (outgoing && (!account || *account == wtx.strFromAccount)) {
+            balance -= debit;
+        }

Nice. utACK 4b49ed65ede769617eab048e4bbab6d5219600c6

I could imagine a follow-up refactoring that get rid of all the individual caches...

    mutable bool fDebitCached;
    mutable bool fCreditCached;
    mutable bool fImmatureCreditCached;
    mutable bool fAvailableCreditCached;
    mutable bool fWatchDebitCached;
    mutable bool fWatchCreditCached;
    mutable bool fImmatureWatchCreditCached;
    mutable bool fAvailableWatchCreditCached;
    mutable bool fChangeCached;
    mutable CAmount nDebitCached;
    mutable CAmount nCreditCached;
    mutable CAmount nImmatureCreditCached;
    mutable CAmount nAvailableCreditCached;
    mutable CAmount nWatchDebitCached;
    mutable CAmount nWatchCreditCached;
    mutable CAmount nImmatureWatchCreditCached;
    mutable CAmount nAvailableWatchCreditCached;
    mutable CAmount nChangeCached;

... and manages them in a std::map or similar.

I could imagine a follow-up refactoring that get rid of all the individual caches...

I also think this could be good to unify/organize these. #9167 adds two more caches, so they could be integrated as well.

Rebased 4b49ed65ede769617eab048e4bbab6d5219600c6 -> e8ce6a4877186940e95e67aeaa12fd11a23b1e0d (getbalance-cleanup.0 -> getbalance-cleanup.1), fixing trivial merge conflict with #9613.

Rebased e8ce6a4877186940e95e67aeaa12fd11a23b1e0d -> be74f34671ac89feaafe80b3ac43ff108c9ceb29 (getbalance-cleanup.1 -> getbalance-cleanup.2) because of conflict with #9764.

Rebased be74f34671ac89feaafe80b3ac43ff108c9ceb29 -> 0b9520d1fb242864b8bd3f6ec68fa91b231467df (pr/getbalance-cleanup.2 -> pr/getbalance-cleanup.3) because of conflicts with pwalletMain renames in #9775.

Needs rebase again (sorry!)

utACK otherwise.

No problem, thanks for the review!

Rebased 0b9520d1fb242864b8bd3f6ec68fa91b231467df -> 02d9f50d5f3c96fe888c230d59c5afdab4c7c6a3 (pr/getbalance-cleanup.3 -> pr/getbalance-cleanup.4). No changes except adding another std:: prefix and passing the right new argument to the CWalletDB constructor.