Change IRC network to Libera Chat

Bump ssb-links

Fix #737

Changed external property in config example. The previous example used an array and led to issues creating invites on the first setup for newbies. The multiserver net plugin seems to call `replace()` on the external property, which does not work for arrays.

Problem: The SSB-Server configures `caps` as a buffer, which breaks all
connections because it's supposed to be a base64 string.

Solution: Use the base64 string just like we're doing in SSB-Config.

ssb-db is not a dev dep

add ssb-private1

Replace `getAddress()` with `address()`

Problem: Removing all `getAddress()` usage was too aggressive -- it's
still a method that Secret-Stack exposes, and so we should still test
for it, but we should **only test for it over in-process method calls**.
When using MuxRPC, we should always use `address()`.

Solution: Use `getAddress()` when the method isn't being called via
MuxRPC.

Problem: Our dependencies don't support Node.js 6 anymore, since its
been EOL since April 2019.

Solution: Drop Travis tests for Node.js 6 and add tests for Node.js 12
and the latest version.

Problem: The `getAddress()` MuxRPC calls seem to be failing, but it's
unclear when the problem started or why. The Secret-Stack documentation
specifies a `getAddress()` method, but it seems to only be available via
an in-process Node.js method call rather than a MuxRPC method call. For
some reason, the MuxRPC method call (which implements the same behavior)
is called `address` rather than `getAddress`.

Solution: Replace all instances of the `getAddress()` MuxRPC method call
with the exact same method call to `address()`. This *is* a breaking
change, but it's a breaking change that already existed in previous
versions of SSB-Server (!) and the next version of SSB-Server needs to
have a major version bump anyway since FlumeDB and SSB-DB have had
breaking changes. This commit also reverts recent changes to
`test/bin.js`, which were causing test failures.