* syncapi: Rename and split out tokens
Previously we used the badly named `PaginationToken` which was
used for both `/sync` and `/messages` requests. This quickly
became confusing because named fields like `PDUPosition` meant
different things depending on the token type. Instead, we now have
two token types: `TopologyToken` and `StreamingToken`, both of
which have fields which make more sense for their specific situations.
Updated the codebase to use one or the other. `PaginationToken` still
lives on as `syncToken`, an unexported type which both tokens rely on.
This allows us to guarantee that the specific mappings of positions
to a string remain solely under the control of the `types` package.
This enables us to move high-level conceptual things like
"decrement this topological token" to function calls e.g
`TopologicalToken.Decrement()`.
Currently broken because `/messages` seemingly used both stream and
topological tokens, though I need to confirm this.
* final tweaks/hacks
* spurious logging
* Review comments and linting
* Fix ordering when backfilling
The problem was that we weren't sorting the returned events
by depth when sending them back to the caller, instead we
were sorting by prev_events which is not the same thing.
* Fixup tests
* Correctly generate backpagination tokens for events which have the same depth
With tests. Unfortunately the code around here is hard to understand.
There will be a subsequent PR which fixes this up now that we have a test
harness in place.
* Add postgres impl
* More linting
* Fix psql statement so it actually works
