reverse_proxy: set default values for keepalive if only some of them are set

[WeidiDeng]

If only some of values are set for http keepalive, the default parameters (keepalive, keepalive_interval, keepalive_idle_conns_per_host) won't be set. This make sure these parameters are set for if other parameters are set.

Assistance Disclosure

No AI was used.

[mholt]

Thanks Weidi!

This reminds me, should we raise the default max idle conns per host, for busy proxies? Seems like it usually improves performance based on your tests.

[WeidiDeng]

I retested using wrk, and found out the performance improvements only comes if the server is already busy (400+ client connections), at 100 connections there is no noticeable improvements.

But I guess when the server is idle, there won't be many idle connections lying around anyway.