Reduce connection struct to 32 bytes

The new scheduler groups two connections per cache line to avoid false
sharing.  For 4 threads, it will schedule each connection (per file
descriptor) like so:

  +---+---|---+---|---+---|---+---|---+---...
  | 0   1 | 2   3 | 4   5 | 6   7 | 8   9 ...
  +---+---|---+---|---+---|---+---|---+---...
  \______/ \_____/ \_____/ \_____/ \_____/
     T 0     T 1     T 2     T 3     T 0

I still haven't tested if this works as intended. Initial testing reduced
the memory as expected (~2MB if set up for 200000 file descriptors), but
also increased the number of instructions (as expected).  Still not sure if
worth it.