Send service wake up push notifications

• Write and benchmark a Lua redis script to fetch contacts about to expire.
  • ~14s to return the entire database (~150000 contacts), ~1s to return the contacts that would expire in the next hour (~1500 contacts) on a local redis instance without load.
• Implement contact fetching on redis
• Implement contact fetching on the internal db
• Test Android client re-register when receiving a notification in background state
• Add field to configure delay between fetches
• Write tests for the ContactExpirationNotifier
• Fix tests not compiling on mac because nghttp2 is missing
• Check Http2Client's behaviour when socket is saturated
• Make ContactExpirationNotifier's lifetime dependent on that of the sofia root for stronger memory safety
• Take Flexisip instance URI as a parameter to filter returned contacts in a clustered configuration
• Fix updatedAt and expires timestamp serialization