move template class member to correct TU for explicit instantiation (#4643)

During recent refactoring a single method of the `template <class
BucketT> class SearchableBucketListSnapshotBase` -- the method
`SearchableBucketListSnapshotBase<BucketT>::loadKeysInternal` -- got
left behind in `SearchableBucketList.cpp` while the rest of the methods
got moved to `BucketListSnapshotBase.cpp`.

Unfortunately all explicit template instantiations of
`SearchableBucketListSnapshotBase` occur in `BucketListSnapshotBase.cpp`
and they only cause instantiation of methods _defined in_ the same TU.
So the method didn't get instantiated. At least on g++.

This was covered up in CI because enabling extrachecks winds up
triggering instantiations anyways. But if you turn off extrachecks,
master doesn't currently compile on g++.

This fixes the bug by moving the method to the right TU.

Author: SirTyson

src/bucket/BucketListSnapshotBase.cpp

@@ -118,6 +118,41 @@ SearchableBucketListSnapshotBase<BucketT>::load(LedgerKey const& k) const
     return result;
 }
 
+template <class BucketT>
+std::optional<std::vector<typename BucketT::LoadT>>
+SearchableBucketListSnapshotBase<BucketT>::loadKeysInternal(
+    std::set<LedgerKey, LedgerEntryIdCmp> const& inKeys,
+    LedgerKeyMeter* lkMeter, std::optional<uint32_t> ledgerSeq) const
+{
+    ZoneScoped;
+
+    // Make a copy of the key set, this loop is destructive
+    auto keys = inKeys;
+    std::vector<typename BucketT::LoadT> entries;
+    auto loadKeysLoop = [&](auto const& b) {
+        b.loadKeys(keys, entries, lkMeter);
+        return keys.empty() ? Loop::COMPLETE : Loop::INCOMPLETE;
+    };
+
+    if (!ledgerSeq || *ledgerSeq == mSnapshot->getLedgerSeq())
+    {
+        loopAllBuckets(loadKeysLoop, *mSnapshot);
+    }
+    else
+    {
+        auto iter = mHistoricalSnapshots.find(*ledgerSeq);
+        if (iter == mHistoricalSnapshots.end())
+        {
+            return std::nullopt;
+        }
+
+        releaseAssert(iter->second);
+        loopAllBuckets(loadKeysLoop, *iter->second);
+    }
+
+    return entries;
+}
+
 template <class BucketT>
 std::optional<std::vector<typename BucketT::LoadT>>
 SearchableBucketListSnapshotBase<BucketT>::loadKeysFromLedger(

src/bucket/SearchableBucketList.cpp

@@ -64,41 +64,6 @@ SearchableLiveBucketListSnapshot::scanForEviction(
     return result;
 }
 
-template <class BucketT>
-std::optional<std::vector<typename BucketT::LoadT>>
-SearchableBucketListSnapshotBase<BucketT>::loadKeysInternal(
-    std::set<LedgerKey, LedgerEntryIdCmp> const& inKeys,
-    LedgerKeyMeter* lkMeter, std::optional<uint32_t> ledgerSeq) const
-{
-    ZoneScoped;
-
-    // Make a copy of the key set, this loop is destructive
-    auto keys = inKeys;
-    std::vector<typename BucketT::LoadT> entries;
-    auto loadKeysLoop = [&](auto const& b) {
-        b.loadKeys(keys, entries, lkMeter);
-        return keys.empty() ? Loop::COMPLETE : Loop::INCOMPLETE;
-    };
-
-    if (!ledgerSeq || *ledgerSeq == mSnapshot->getLedgerSeq())
-    {
-        loopAllBuckets(loadKeysLoop, *mSnapshot);
-    }
-    else
-    {
-        auto iter = mHistoricalSnapshots.find(*ledgerSeq);
-        if (iter == mHistoricalSnapshots.end())
-        {
-            return std::nullopt;
-        }
-
-        releaseAssert(iter->second);
-        loopAllBuckets(loadKeysLoop, *iter->second);
-    }
-
-    return entries;
-}
-
 // This query has two steps:
 //  1. For each bucket, determine what PoolIDs contain the target asset via the
 //     assetToPoolID index