CLEANUP: proxy: slightly reorganize fields to plug some holes

CLEANUP: proxy: slightly reorganize fields to plug some holes

The proxy struct has several small holes that deserved being plugged by
moving a few fields around. Now we're down to 3056 from 3072 previously,
and the remaining holes are small.

At the moment, compared to before this series, we're seeing these
sizes:

    type\size   7d554ca62   current  delta
    listener       752        704     -48  (-6.4%)
    server        4032       3840    -192  (-4.8%)
    proxy         3184       3056    -128  (-4%)
    stktable      3392       3328     -64  (-1.9%)

Configs with many servers have shrunk by about 4% in RAM and configs
with many proxies by about 3%.