During IBD, when doing pruning, prune 10% extra to avoid pruning agai…

…n soon after

Summary:
Pruning forces a chainstate flush, which can defeat the dbcache and harm performance significantly.

Backport of Bitcoin Core PR11658
bitcoin/bitcoin#11658

Test Plan:
```
make check
```

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3986

src/validation.cpp

@@ -4297,6 +4297,16 @@ static void FindFilesToPrune(std::set<int> &setFilesToPrune,
     int count = 0;
 
     if (nCurrentUsage + nBuffer >= nPruneTarget) {
+        // On a prune event, the chainstate DB is flushed.
+        // To avoid excessive prune events negating the benefit of high dbcache
+        // values, we should not prune too rapidly.
+        // So when pruning in IBD, increase the buffer a bit to avoid a re-prune
+        // too soon.
+        if (IsInitialBlockDownload()) {
+            // Since this is only relevant during IBD, we use a fixed 10%
+            nBuffer += nPruneTarget / 10;
+        }
+
         for (int fileNumber = 0; fileNumber < nLastBlockFile; fileNumber++) {
             nBytesToPrune = vinfoBlockFile[fileNumber].nSize +
                             vinfoBlockFile[fileNumber].nUndoSize;