PERF: Add exponential backoff for DistributedMutex (#17886)

Polling every 0.001s can cause extreme load on the redis instance, especially in scenarios where multiple app instances are waiting on the same lock. This commit introduces an exponential backoff starting from 0.001s and reaching a maximum interval of 1s.

Previously `CHECK_READONLY_ATTEMPTS` was 10, and resulted in a block for 0.01s. Under the new logic, 10 attempts take more than 1s. Therefore CHECK_READONLY_ATTEMPTS is reduced to 5, bringing its total time to around 0.031s

lib/distributed_mutex.rb

@@ -4,7 +4,7 @@
 # Expiration happens when the current time is greater than the expire time
 class DistributedMutex
   DEFAULT_VALIDITY = 60
-  CHECK_READONLY_ATTEMPTS = 10
+  CHECK_READONLY_ATTEMPTS = 5
 
   LOCK_SCRIPT = DiscourseRedis::EvalHelper.new <<~LUA
     local now = redis.call("time")[1]
@@ -85,7 +85,9 @@ class DistributedMutex
 
       return expire_time if expire_time
 
-      sleep 0.001
+      # Exponential backoff, max duration 1s
+      interval = attempts < 10 ? (0.001 * 2**attempts) : 1
+      sleep interval
 
       # in readonly we will never be able to get a lock
       if @using_global_redis && Discourse.recently_readonly?