CPUThreadPoolExecutor's default queue is quite big

CPUThreadPoolExecutor's default queue is quite big

Summary:
By default, CPUThreadPoolExecutor uses 100 MB of rss for the CPU queue. This is
substantially higher than expected.

Reduce the default queue size so that it uses much less memory upfront, down
to about 6 MB going by the heap profile.

Reviewed By: davejwatson@fb.com

Subscribers: mwa, ott, iaroslav, jrsherwood, aflock, slarsen, davejwatson, trunkagent, fugalh, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1900539

Tasks: 6453427

Signature: t1:1900539:1426103805:34fb7b081058fbde89370b5d9ec24ea572c4b0e2