validation: randomly compact chainstate

Full chainstate flushes are convenient maintenance points for long-term LevelDB cleanup because the chainstate was just written.
Randomize the trigger so nodes that flush near the same height do not compact together.

Add blocking chainstate compaction through `CCoinsViewDB::CompactFull()` and give each post-IBD full flush on the normal chainstate a 1/320 chance to start compaction.
With hourly flushes this averages roughly every two weeks and makes a six-month miss about one in a million.
This keeps the schedule stateless and leaves last-compaction height or timestamp bookkeeping out of chainstate metadata.

Co-authored-by: Andrew Toth <andrewstoth@gmail.com>

src/dbwrapper.cpp

@@ -256,7 +256,7 @@ CDBWrapper::CDBWrapper(const DBParams& params)
 
     if (params.options.force_compact) {
         LogInfo("Starting database compaction of %s", fs::PathToString(params.path));
-        DBContext().pdb->CompactRange(nullptr, nullptr);
+        CompactFull();
         LogInfo("Finished database compaction of %s", fs::PathToString(params.path));
     }
 
@@ -307,6 +307,8 @@ std::optional<std::string> CDBWrapper::GetProperty(const std::string& property)
     return std::nullopt;
 }
 
+void CDBWrapper::CompactFull() { DBContext().pdb->CompactRange(nullptr, nullptr); }
+
 size_t CDBWrapper::DynamicMemoryUsage() const
 {
     std::optional<size_t> parsed;