MINOR: server: align server struct to 64 bytes

MINOR: server: align server struct to 64 bytes

Several times recently, it was noticed that some benchmarks would
highly vary depending on the position of certain fields in the server
struct, and this could even vary between runs.

The server struct does have separate areas depending on the user cases
and hot/cold aspect of the members stored there, but the areas are
artificially kept apart using fixed padding instead of real alignment,
which has the first sad effect of artificially inflating the struct,
and the second one of misaligning it.

Now that we have all the necessary tools to keep them aligned, let's
just do it. The struct has shrunk from 4160 to 4032 bytes on 64-bit
systems, 152 of which are still holes or padding.