1. Remove in-memory cache. It doesn't persist objects and if we want
more speed, `NoSync` option can be used for the bolt DB.
2. Put to the metabase in a synchronous fashion. This considerably
simplifies overall logic and plays nicely with the metabase bolt DB
batch settings.
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
`Degraded` mode is set automatically after error counter is over the
threshold. `ReadOnly` mode can still be set by an administrator.
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
In read-only mode modifying operations are immediately returned with
error and all background operations are suspended.
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
