Prevent race condition on load balancer fix (final)

What does this MR do and why?

Replace redis set with a lua script to atomically update the LSN to prevent race conditions on the db load balancer. See issue below for important details.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/573726

Screenshots or screen recordings

Before	After

How to set up and validate the LUA script locally:

you can save the ATOMIC_STICKING_SCRIPT into a file (i.e. script.lua) and go to the gkd/redis/ directory:

1. Make sure redis is running (gdk start redis)
2. Examples:

# First write (no existing key) — returns (integer) 1
redis-cli -s redis.socket --eval script.lua mylsnkey , 0/1000 30

# Higher LSN — returns (integer) 1, updates value
redis-cli -s redis.socket --eval script.lua mylsnkey , 0/2000 30

# Lower LSN — returns (integer) 0, keeps current, refreshes TTL
redis-cli -s redis.socket --eval script.lua mylsnkey , 0/1500 30

# Invalid LSN — returns (error) ERR ARGV[1] must be a valid LSN (e.g. 0/16B3A78)
redis-cli -s redis.socket --eval script.lua mylsnkey , 120 30

# Invalid TTL — returns (error) ERR ARGV[2] (TTL) must be a positive integer
redis-cli -s redis.socket --eval script.lua mylsnkey , 0/1000 0

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