Node 8.7.2 update and misc improvements

[johnalotoski]

• Increasing high load relays to ~100% p2p over time
• Bumps db-sync snapshots to 13.1.1.3
• Bumps node deploy on relays to 8.7.2 release, and iohk-nix to master for node 8.7.2 config
• Sets a mainnet legacy network topology relay canary and a reduced RAM relay canary
• Increases the monitoring scrape timeout to 60 seconds to avoid transient scrape errors
• Pins relays-new definition for mainnet to the proper legacy name: relays-new.cardano-mainnet.iohk.io to avoid conflict with the iohk-nix master backbone.cardano-mainnet.iohk.io which is appropriate for machines outside the cardano-ops cluster.
• Adjust the latency alert threshold
• Pin the snapshots dbsync machine to node 8.1.2 until the next dbsync release and adjust systemd timeouts
• Adjust the metadata server rate limit and nginx return byte logging
• Add a deployment note for the migrated legacy mainnet pools