[DH-3] tweaking core pool sysctl tcp settings... again

[shaneknapp]

after a year, and after seeing more SYN flooding... as well as reading more and understanding the variables a bit more i wanted to revisit #4488

the big takeaways here are:

1. we have more ram (64G n2-highmem-8 vs 8G n2-standard-8), and should consider allocating a bit more to the kernel for tcp
2. our max number of connections (!!!!) was only 1024 (!?!?!?!). i increased this to 4096 (somaxconn) which i think might help
3. our nodes look to have lots and lots of bandwidth, so let's take advantage of that! (https://cloud.google.com/compute/docs/network-bandwidth#egress)

this can safely be merged at any time, and whatever comes out of this PR will be used when we create a new core pool w/more cpu (as 8 cores isn't enough when you run as many hubs as we do on a node) and the same ram.

a bunch of these settings tweaks came from https://fasterdata.es.net/host-tuning/linux/#toc-anchor-2

@ryanlovett @consideRatio @yuvipanda any thoughts?