metabase.Open() now reports metabase mode metric. shard.UpdateID()
needs to read shard ID from metabase => needs to open metabase.
It caused reporting 'shard undefined' metrics. To avoid reporting
wrong metrics metabase.GetShardID() was added which also opens
metabase and does not report metrics.
Signed-off-by: Ekaterina Lebedeva <ekaterina.lebedeva@yadro.com>
Includes:
1. mode change read lock operation in every exported method that r/w the
underlying database;
2. returning `ErrDegradedMode` logical error if any exported method is
called in degraded (without a metabase) mode.
Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>
