* Remove `conn_id` usage for UserSyncCommand.

Each tcp replication connection is assigned a "conn_id", which is used
to give an ID to a remotely connected worker. In a redis world, there
will no longer be a one to one mapping between connection and instance,
so instead we need to replace such usages with an ID generated by the
remote instances and included in the replicaiton commands.

This really only effects UserSyncCommand.

* Add CLEAR_USER_SYNCS command that is sent on shutdown.

This should help with the case where a synchrotron gets restarted
gracefully, rather than rely on 5 minute timeout.

That fallback sets the redirect URL to itself (so it can process the login
token then return gracefully to the client). This would make it pointless to
ask the user for confirmation, since the URL the confirmation page would be
showing wouldn't be the client's.

Signed-off-by: Aaron Raimist <aaron@raim.ist>

This changes the replication protocol so that the server does not send down `RDATA` for rows that happened before the client connected. Instead, the server will send a `POSITION` and clients then query the database (or master out of band) to get up to date.

* Pull Sentinel out of LoggingContext

... and drop a few unnecessary references to it

* Factor out LoggingContext.current_context

move `current_context` and `set_context` out to top-level functions.

Mostly this means that I can more easily trace what's actually referring to
LoggingContext, but I think it's generally neater.

* move copy-to-parent into `stop`

this really just makes `start` and `stop` more symetric. It also means that it
behaves correctly if you manually `set_log_context` rather than using the
context manager.

* Replace `LoggingContext.alive` with `finished`

Turn `alive` into `finished` and make it a bit better defined.

Attempts to clarify the sample config for databases, and add some stuff about
tcp keepalives to `postgres.md`.

fix broken link

This reverts commit 54dd2862, reversing
changes made to 6640460d.

 * Give `notif_template_html`, `notif_template_text` default values (fixes #6960)
 * Don't complain if `smtp_host` and `smtp_port` are unset, since they have sensible defaults (fixes #6961)
 * Set the example for `enable_notifs` to `True`, for consistency and because it's more useful
 * Raise errors as ConfigError rather than RuntimeError for nicer formatting

Ensure good comprehension hygiene using flake8-comprehensions.

Fixes #6916.

Add a method to the spam checker to filter the user directory results.

Add documentation for the spam checker.