Expose request and don't assume sbot.replicate is available

[arj03]

This PR does two things that I think are close enough related to put into one:

• Add request method. This is actually documented but is never exposed.
• Handles sbot.replicate is not available, as one might have a close-enough-to-sbot that doesn't use ssb-replicate. The code more or less handles this, so it seems rather natural that ssb-ebt should be able to load without ssb-replicate.

The motivation for these changes is that I need them for my ssb-browser project.

[christianbundy]

Looks good to me, feel free to merge.

[dominictarr]

I think there is a slightly deeper problem here: replicate.request is the interface that is needed by any plugins that have opinions about what feeds to replicate. similar to gossip.add (for peers to replicate with) if we change the module that provides that service, we don't want to update all the modules that use it... because that will interfere with exploring different designs in parallel, or upgrading to new versions of modules while other people are still using the old one.

[arj03]

Right, one needs to use the replication interface.

I think the coupling is a bit wierd in that the new module depends on the old, also that the description of ssb-replicate is "ssb legacy replication", so it sounds as if it is only legacy. Anyway will close this. Thanks for clearing that up.