Refactor StorageSessionManager to load proxy info from DB

Refactor StorageSessionManager (session_manager.py) to create StorageProxyInfo objects by querying the DB instead of reading etcd. This is the core read-path switch for storage proxy metadata.

JIRA Issue: BA-4504

Start with session_manager.py and trace how StorageSessionManager currently reads proxy metadata from etcd and where StorageProxyInfo objects are created. The work is done when proxy info is queried from the DB instead and the read path no longer depends on etcd for this metadata; the issue does not name a test to run.

Written by the indexing model from the issue text.