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.