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Also, make the libp2p constructor fully useful. There should now be no need to
manually construct a swarm/host.

This way, we actually process disconnect notifications (and reduce the lifetime
on peer addr records).

This commit prevents us from repeatedly extending the lifetimes of all observed
addresses if a peer keeps on reconnecting.

It also fixes two race conditions:

1. We may end up processing a disconnect after a re-connect and may
accidentally give the addresses associated with that peer a
RecentlyConnectedAddrTTL instead of a ConnectedAddrTTL.

2. We may end up processing a connect after the associated disconnect storing
the associated peer addresses indefinitely.

There were previously 4 different ways of passing various options
into BasicHost construction.

1. Function parameters to New
2. Option parameters to New
3. Global variables, e.g. NegotiateTimeout
4. Options struct, in the calling code in go-ipfs

This changeset deprecated all of these, and introduces the HostOpts
struct to replace them. It also introduces a new constructor called
NewHost, which accepts a *HostOpts.

The old New constructor continues to work the same way,
but is from now on deprecated too.

Make sure to verify that our keys match.
In the rare event they do not, make sure to investigate
what's wrong, and log errors.