R1:5978bbfc64e2

Do sampled profiling of requests

Summary:
People have occasionally complained about phabricator being slow. We have
the access log to look at to see when slowness happens, but it doesn't tell
us much about why it happened. Since it's usually a sporadic issue that's
reported, it's hard to reproduce and then profile. This change will allow us
to collect sampled profiles so we can look at them when slowness occurs.

Test Plan:
checking that sampling works correctly:
- set rate to 0; do several page loads…