Add /state_ids federation API

Port script: Handle the fact that some tables have negative rowid rows

keys/query: Omit device displayname if null

* Factor out more common code from the jenkins scripts

* Fix install_and_run path

* Poke jenkins

* Poke jenkins

... which makes it more consistent with user displaynames.

Include device name in /keys/query response

keys/query: return all users which were asked for

Add an 'unsigned' section which includes the device display name.

In the situation where all of a user's devices get deleted, we want to
indicate this to a client, so we want to return an empty dictionary, rather
than nothing at all.

The new API only returns the event_ids for the state, as most
requesters will already have the vast majority of the events already.

PR feedback

Ensure we only persist an event once at a time

It turns out that it's more useful to return a null device display name (and
let clients decide how to handle it: eg, falling back to device_id) than using
a constant string like "unknown device".

* Factor out some of the code shared between the different sytest jenkins scripts

* Exclude jenkins from the MANIFEST

* Fix dendron build

* Missing new line

* Poke jenkins

* Export the PORT_BASE and PORT_COUNT

* Poke jenkins

Using XPath is slow on some machines (for unknown reasons), so use a
different approach to get a list of text nodes.

Try to generate a summary that respect paragraph and then word
boundaries, adding ellipses when appropriate.

Refactor the e2e query handler to separate out the local query, and then make
the federation handler use it.