... and rename it, for even more sanity

We don't do anything with the result, so this is needed to give this code a
logcontext.

This brings it into line with on_new_notifications and on_new_receipts. It
requires a little bit of hoop-jumping in EmailPusher to load the throttle
params before the first loop.

`on_new_notifications` and `on_new_receipts` in `HttpPusher` and `EmailPusher`
now always return synchronously, so we can remove the `defer.gatherResults` on
their results, and the `run_as_background_process` wrappers can be removed too
because the PusherPool methods will now complete quickly enough.

Each pusher has its own loop which runs for as long as it has work to do. This
should run in its own background thread with its own logcontext, as other
similar loops elsewhere in the system do - which means that CPU usage is
consistently attributed to that loop, rather than to whatever request happened
to start the loop.

simplifies the interface to _start_pushers

... and use it from start_pusher_by_id. This mostly simplifies
start_pusher_by_id.

This is public (or at least, called from outside the class), so ought to have a
better name.

This fallback didn't work, and was removed in #4069.

Calculate absolute path for email templates

Make manhole work on Python 3 again

Use the right python when starting workers

We should use the same python to start the workers as we do for the main
synapse (ie, the same one used to run synctl.)

this change has been released

Only colourise synctl output when attached to tty

It's quite important that get_missing_events returns the *latest* events in the
room; however we were pulling event ids out of the database until we got *at
least* 10, and then taking the *earliest* of the results.

We also shouldn't really be relying on depth, and should be checking the
room_id.

Ship the email templates as package_data

Fix bug where we raised StopIteration in a generator

Assume isatty is always defined, and catch AttributeError. Also don't bother checking colour==Normal

move the example email templates into the synapse package so that they can be
used as package data, which should mean that all of the packaging mechanisms
(pip, docker, debian, arch, etc) should now come with the example templates.

In order to grandfather in people who relied on the templates being in the old
place, check for that situation and fall back to using the defaults if the
templates directory does not exist.

This made python 3.7 unhappy

Thanks @Half-Shot !!!