Adding proxy.config.http.allow_half_open

[zizhong]

This is a workaround for issue #3288.

The root cause of this issue is when ATS gets RST in the state of half-close, the read side is closed and the RST is ignored.
#947 is trying to bring up the RST to HttpSM as a VIO error event, but the SM doesn't expect an event on a disabled read, so the SM ends up crashing.

This PR is to give a workaround with little risk until we figure out what the proper way to fix #3288.

@bryancall @zwoop @oknet @scw00 any ideas would be much appreciated.

[scw00]

Looks good ! thanks

[scw00]

Hi @zwoop @bryancall
This pr make half-closed configurable, but I'm not sure whether we should launch this pr . Could you please review again ?

[oknet]

Please inform the end user that it is dangerous to turn it off. It can only be turned off in special scenario.

[bryancall]

We plan to talk about half open at the summit this Spring. Please hold off merging this till after.

[shinrich]

The implementation looks fine to me. Should probably rename it to allow_half_closed if we decide to go with this. Probably my fault. I added allow_half_open method to the ProxyClient* classes. In the general community it seems that half open refers to embryonic connections (got the SYN but not the SYN-ACK). And half closed refers to this situation. Most of the code refers to half_close_flag's, so I just had a brain bubble when naming my method.

[shinrich]

In the discussion at the Euro Tour, the consensus was to add this flag and during the 8.0 time frame add metrics to figure out how often this really happens for various scenarios.