Improve performance for syncing many small files quickly

Without this, as soon as we'd touched 1000 files in the last minute
(which can happen), we got stuck doing cache cleaning all the time,
burning a lot of CPU time.

internal/model/model.go

@@ -808,6 +808,20 @@ func (m *Model) Request(deviceID protocol.DeviceID, folder, name string, offset
 					delete(m.reqValidationCache, name)
 				}
 			}
+
+			if len(m.reqValidationCache) > reqValidationCacheSize*9/10 {
+				// The first clean didn't help much, we're still over 90%
+				// full; we may have synced a lot of files lately. Prune the
+				// cache more aggressively by removing every other item so we
+				// don't get stuck doing useless cache cleaning.
+				i := 0
+				for name := range m.reqValidationCache {
+					if i%2 == 0 {
+						delete(m.reqValidationCache, name)
+					}
+					i++
+				}
+			}
 		}
 		m.rvmut.Unlock()
 	}