remove artifical tcp send/recv buffer limit, and add threads for io..

[jmg]

Summary

In doing testing on a VM a long while ago, I noticed that ggate
was forcing the recv/send buffer to be 128k by default.. Now that
FreeBSD has buffer autosizing, this was restricting how much data
could be pending in the buffer at a time..

Also, ggated serializes IO, even though the IO's should be able
to be executed in parallel, and ggated is already multithreaded
for the request processing and IO side, so simply start up more
threads..