Add ability to resolve (and cache) multicast groups for a given family name

[Sh3Rm4n]

Continuation of #12

[cathay4t]

If this multicast group is also in GetFamily reply, I think we should merge this function into query_family_id(), so we have single netlink request to get both caches ready.

[Sh3Rm4n]

Good point. But in that case query_family_ids function signature does not really fit anymore.

What about

fn query_family(&mut self, handle: &GenetlinkHandle, family_name: &str) -> (u16, HashMap<String, u32>)

which returns both id and the groups. At this point, I should probably introduce a struct instead of the tuple from above.

We could still keep query_family_id (and query_family_multicast_groups, which would call query_family internally for backwards compatibility. WDTY?

[cathay4t]

@Sh3Rm4n ping

[Sh3Rm4n]

I've unified the interface based on #16 (comment)

Instead of removing, I've deprecated obsolete functions, so that this PR is not a breaking change. If this doesn't matter, I could remove those deprecated functions as well.

[cathay4t]

I have amended the patch:

1. Squash into single commit.
2. Fixed the test code to use new query_family() instead of deprecated query_family_id().