Listener for events of metastore lifecycle.
Database manager is responsible for initializing metastore on node startup
and notifying other components about its readiness.
Is called when metastorage is made ready for read-only operations very early on node startup phase.
Reference for read-only metastorage should be used only within this method and shouldn't be stored
to any field.
Fully functional metastore capable of performing reading and writing operations.
Components interested in using metastore are allowed to keep reference passed into the method
in their fields.