fix: missing read lock in MapWithTTL

[VinozzZ]

Which problem is this PR solving?

fatal error: concurrent map read and map write

goroutine 193 [running]:
github.com/honeycombio/refinery/generics.(*MapWithTTL[...]).SortedValues(0xdd8a60)
	github.com/honeycombio/refinery/generics/mapttl.go:118 +0xcc
github.com/honeycombio/refinery/internal/peer.(*RedisPubsubPeers).Ready.func1()
	github.com/honeycombio/refinery/internal/peer/pubsub_redis.go:212 +0x454
created by github.com/honeycombio/refinery/internal/peer.(*RedisPubsubPeers).Ready in goroutine 1
	github.com/honeycombio/refinery/internal/peer/pubsub_redis.go:180 +0x84

Short description of the changes

• add RLock for SortedValues

[kentquirk]

I forgot that indexing Items needed a lock.

This solution uses two lock cycles (one to get the sorted keys, and a second one you add here).

I think that's preferable to a single lock because the sort takes place without holding the lock. So yeah, let's go with this.