Add a sysctl to toggle the use of the sockets LOWAT when calculating auto window growth

[j-nitrology.com]

Summary

For applications that don't send as aggressively, this helps ensure we still grow the local window by taking its low watermark into account. By default, SO_SNDLOWAT is a negligable 1024. When this number is bumped up for efficiency on a large number of sessions, it starts to affect the ability for growth in the current code.

Test Plan

We have been running with this SO_SNDLOWAT modification at LLNW for a few years

[j-nitrology.com]

It may be more proper for this to go into tflag, but the downsides there is then it won't apply to existing sessions and we consume more space (which is low with the current padding). Open to any opinions, as locally we just have it hard coded on in any case.

[hiren]

I know this has been in prod for quite some time and works. I don't want to engage in "best-practices" for this. :-) LGTM.