Make gateway HA fallback immediate

[j-chmielewski]

Follow this scenario https://docs.defguard.net/2.0/deployment-strategies/previewing-defguard-v2.0-alpha#test-ha-for-gateway-components

When one of the gateways becomes unavailable, wireguard tunnel stops working for noticeable time. This degrades the failover experience and can cause temporary loss of connectivity.

We should investigate what causes this delay and, if possible (@wojcik91?), make the fallback to a healthy gateway happen immediately (or at least significantly faster).

[kchudy]

• Also check if stats work in Failover scenarios.
• Test if it works with HAProxy or other LB, recommend a working load balancer
• Test various LB strategies
• Document how handshake occurs (point to wireguard docs)

[moubctez]

Docs https://www.wireguard.com/protocol/

[j-chmielewski]

PR: DefGuard/deployment#131
Docs:

• https://docs.defguard.net/2.0/deployment-strategies/high-availability-and-failover#gateway-high-availability
• https://docs.defguard.net/2.0/deployment-strategies/high-availability-and-failover/wireguard-udp-load-balancing

[j-chmielewski]

Dev environment deployment description: https://kb.defguard.net/doc/dev-environment-gateway-and-firewall-Ntn4ydRikY