master: Add constructor and destructor for FilesystemMetadata

This makes it possible to free memory occupied by the metadata when
metadata server is running to load other metadata file.

Change-Id: I95f0a8a9f17895444f65fae8ee1ece0b89288114
master: Reset session timeouts when shadow becomes master

Shadow doesn't keep track of session connections and disconnections.
All sessions appear to have been disconnected immediately after their
creation.

Reset session disconnection timestamps when becoming a master to avoid
immediate deletion of sessions created long time ago.

Change-Id: I8a38437a69cb1ee576862c2a239e25869b853d30