[feature] Prune timelines once per hour to plug memory leak (#1117)

* export highest/lowest ULIDs as proper const

* add stop + start to timeline manager, other small fixes

* unexport unused interface funcs + tidy up

* add LastGot func

* add timeline Prune function

* test prune

* update lastGot

internal/timeline/prune.go

@@ -0,0 +1,68 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package timeline
+
+import (
+	"container/list"
+)
+
+const (
+	defaultDesiredIndexedItemsLength  = 400
+	defaultDesiredPreparedItemsLength = 50
+)
+
+func (t *timeline) Prune(desiredPreparedItemsLength int, desiredIndexedItemsLength int) int {
+	t.Lock()
+	defer t.Unlock()
+
+	pruneList := func(pruneTo int, listToPrune *list.List) int {
+		if listToPrune == nil {
+			// no need to prune
+			return 0
+		}
+
+		unprunedLength := listToPrune.Len()
+		if unprunedLength <= pruneTo {
+			// no need to prune
+			return 0
+		}
+
+		// work from the back + assemble a slice of entries that we will prune
+		amountStillToPrune := unprunedLength - pruneTo
+		itemsToPrune := make([]*list.Element, 0, amountStillToPrune)
+		for e := listToPrune.Back(); amountStillToPrune > 0; e = e.Prev() {
+			itemsToPrune = append(itemsToPrune, e)
+			amountStillToPrune--
+		}
+
+		// remove the entries we found
+		var totalPruned int
+		for _, e := range itemsToPrune {
+			listToPrune.Remove(e)
+			totalPruned++
+		}
+
+		return totalPruned
+	}
+
+	prunedPrepared := pruneList(desiredPreparedItemsLength, t.preparedItems.data)
+	prunedIndexed := pruneList(desiredIndexedItemsLength, t.indexedItems.data)
+
+	return prunedPrepared + prunedIndexed
+}