[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/typeutils/internaltoas.go

@@ -34,13 +34,6 @@ import (
 	"github.com/superseriousbusiness/gotosocial/internal/log"
 )
 
-// const (
-// 	// highestID is the highest possible ULID
-// 	highestID = "ZZZZZZZZZZZZZZZZZZZZZZZZZZ"
-// 	// lowestID is the lowest possible ULID
-// 	lowestID = "00000000000000000000000000"
-// )
-
 // Converts a gts model account into an Activity Streams person type.
 func (c *converter) AccountToAS(ctx context.Context, a *gtsmodel.Account) (vocab.ActivityStreamsPerson, error) {
 	person := streams.NewActivityStreamsPerson()