Increase SIZE_POOL_COUNT to 5

Having more size pools will allow us to allocate larger objects
through Variable Width Allocation.

I have attached some benchmark results below.

Discourse:
  On Discourse, we don't see much change in response times. We do see
  a small reduction in RSS.

  Branch RSS: 377.8 MB
  Master RSS: 396.3 MB

railsbench:
  On railsbench, we don't see a big change in RPS or p99 performance.
  We see a small increase in RSS.

  Branch RPS: 815.38
  Master RPS: 811.73

  Branch p99: 1.69 ms
  Master p99: 1.68 ms

  Branch RSS: 90.6 MB
  Master RSS: 89.4 MB

liquid:
  We don't see a significant change in liquid performance.

  Branch parse & render: 29.041 I/s
  Master parse & render: 29.211 I/s

0 files changed, 0 insertions, 0 deletions