FR: Add an expiring Multimap

[Dietr1ch]

Would you consider adding an expiring multimap?

A multimap allows multiple values for a given key, so it behaves like a HashMap<K, HashSet> with a nice interface to iterate over values for a given key, but when considering expiration that trick doesn't behave nicely as expiration should be per value, and not per key and shared across its values.

My use case is holding DNS/mDNS records, where for instance in address records, you'll have (hostname, IP) entries with different TTLs, and possibly multiple IPs for the same hostname (IPv4, IPv6, public and private IPs).

[ArtBlnd]

Hello, @Dietr1ch

Sorry for the late response. I've busy these days.
I'll try to implement multimap if it is possible on current design.