Notification expiry - compact existing notifications and improve compaction strategy

[murali-shris]

• strategy to compact existing notifications
• improve compaction strategy eg. 100 notifications and only 1 notification to compact.Skip compaction in this scenario
• for old notifications, compact based on notification creation date

[murali-shris]

PR for compacting old notifications https://github.com/atsign-foundation/at_server/pull/442/files

[murali-shris]

server released to canaries