[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/id/ulid.go

@@ -1,3 +1,21 @@
+/*
+   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 id
 
 import (
@@ -8,7 +26,11 @@ import (
 	"github.com/oklog/ulid"
 )
 
-const randomRange = 631152381 // ~20 years in seconds
+const (
+	Highest     = "ZZZZZZZZZZZZZZZZZZZZZZZZZZ" // Highest is the highest possible ULID
+	Lowest      = "00000000000000000000000000" // Lowest is the lowest possible ULID
+	randomRange = 631152381                    // ~20 years in seconds
+)
 
 // ULID represents a Universally Unique Lexicographically Sortable Identifier of 26 characters. See https://github.com/oklog/ulid
 type ULID string