The cache is only approximate, i.e. it doesn't skip edits and similar,
but this might be good enough? Also no migration right now.

Speeds up startup by about 5x on my system.

Half the startup time is now loading the powerlevels for each room. We
can probably lazily load those too in the future.

implements MSC3700

see https://github.com/matrix-org/matrix-spec-proposals/pull/3700

fixes #873

fixes #857

This change adds the ability to run Nheko without a secrets storage
service[1]. The secrets are stored in plaintext under the [secrets]
section of nheko.conf. Since this is a "I know what I'm doing" type
of feature, it's disabled by default and can only be enabled with an
hidden setting `run_without_secure_secrets_service=true`.

Fixes issue #705.

[1]: https://freedesktop.org/wiki/Specifications/secret-storage-spec