Set Server header in SynapseRequest

transaction_id, destination defined twice

(instead of everywhere that writes a response. Or rather, the subset of places
which write responses where we haven't forgotten it).

This also means that we don't have to have the mysterious version_string
attribute in anything with a request handler.

Unfortunately it does mean that we have to pass the version string wherever we
instantiate a SynapseSite, which has been c&ped 150 times, but that is code
that ought to be cleaned up anyway really.

Refactor request handling wrappers

This is needless complexity; we might as well use the wrapper directly.

Also rename wrap_request_handler->wrap_json_request_handler.

... so that it can be used on non-JSON endpoints

The metrics are now available via the request, so this is redundant and can go
away at last.

it's much neater there.

less magic

It fits quite nicely here, and opens the path to getting rid of the
"include_metrics" mess.

... which is going to make it easier to move around.

This is useful in its own right, because server.py is full of stuff; but more
importantly, I want to do some refactoring that will cause a circular reference
as it is.

Refactor sync APIs to reuse pagination API

Refactor get_recent_events_for_room return type

 Remove unused code path from member change DB func

 Refactor pagination DB API to return concrete type

The function is never called without a from_key, so we can remove all
the handling for that scenario.

 Refactor recent events func to use pagination func

The sync API often returns events in a topological rather than stream
ordering, e.g. when the user joined the room or on initial sync. When
this happens we can reuse existing pagination storage functions.

This allows calling the `get_recent_event_ids_for_room` function in more
situations.

nuke-room-from-db.sh: fix deletion from search table

There is no reason to return a tuple of tokens when the last token is
always the token passed as an argument. Changing it makes it consistent
with other storage APIs

This makes it easier to document what is being returned by the storage
functions and what some functions expect as arguments.

This also removes a cache that is unlikely to ever get hit.

Refactor /context to reuse pagination storage functions

nuke-room-from-db.sh: added postgresql option and help