Latency monitor trheshold value is now configurable.

This commit adds both support for redis.conf and CONFIG SET/GET.

redis.conf

@@ -562,6 +562,27 @@ slowlog-log-slower-than 10000
 # You can reclaim memory used by the slow log with SLOWLOG RESET.
 slowlog-max-len 128
 
+################################ LATENCY MONITOR ##############################
+
+# The Redis latency monitoring subsystem samples different operations
+# at runtime in order to collect data related to possible sources of
+# latency of a Redis instance.
+#
+# Via the LATENCY command this information is available to the user that can
+# print graphs and obtain reports.
+#
+# The system only logs operations that were performed in a time equal or
+# greater than the amount of milliseconds specified via the
+# latency-monitor-threshold configuration directive. When its value is set
+# to zero, the latency monitor is turned off.
+#
+# By default latency monitoring is disabled since it is mostly not needed
+# if you don't have latency issues, and collecting data has a performance
+# impact, that while very small, can be measured under big load. Latency
+# monitoring can easily be enalbed at runtime using the command
+# "CONFIG SET latency-monitor-threshold <milliseconds>" if needed.
+latency-monitor-threshold 0
+
 ############################# Event notification ##############################
 
 # Redis can notify Pub/Sub clients about events happening in the key space.