write_http: make callback names context-dependent

write_http: make callback names context-dependent

This allows multiple destinations to work again (fixes #821), using the
same logic as other write plugins.

The callback name would now be something like:
    `write_http/http://example.com/endpoint`
which is not very nice.

The next step would be to change this plugin to use `<Node>` blocks
like many others, and pass the URL as a parameter inside each instance
block. So I see this patch as the minimum required to let 5.3 and 5.4
users use this plugin with multiple destinations.