Test: AOF rewrite during write load.
Test: enable latency monitor in the default config.

This way as a side effect of running the test we also stress the latency
monitor data collection.
LATENCY HISTORY returns empty array if no data is available.

Previously we returned an error.
LATENCY command / monitor basic tests.
Test: more reliable AOF rewrite test under write load.
Merge remote-tracking branch 'upstream/unstable' into unstable
Redis epoll 分析
Redis 网络层分析