Add timestamp selection of events

Referring to the conversation following
https://matrix.to/#/!XqBunHwQIXUiqCaoxq:matrix.org/%2414992263384597835OhNlB:matrix.org
and
https://matrix.to/#/!XqBunHwQIXUiqCaoxq:matrix.org/%241499257197105mxkVo:jki.re

> I'm proposing the ability to set start and end to timestamps as well as the existing sequence ids.

> I was thinking using the same fields but prefixing the timestamp with 'ts' would probably be enough, although adding 2 new fields "start_ts" and "end_ts" is also an option.
NOTE: Ralith correctly suggests the new fields are a better solution

> My main interest is the message endpoint.
Being able to set specific start and end timestamps would significantly ease scripted retrieval of messages.
Essentially allowing no state to be stored between script runs (eg: a script which runs from cron and grabs the last hours messages. Where nothing older than an hour matters at all)

Read the two linked Matrix discussions, then inspect the message endpoint specification and its handling of sequence-based start and end values. Define support for separate start_ts and end_ts parameters, including their expected behavior, and update the endpoint specification and related examples or tests if present.

Written by the indexing model from the issue text.