Two optimizations to singificantly speed up some collectors.

- Added option to prefilter the ingestion set to prevent repeated
  parsers.
- Added option to enforce_hint for collectors that are extremely broad
  but frequently called for very specific things.

The first optimization speeds up netstat as follows:
Before:
21752060 function calls (20738464 primitive calls) in 21.943 seconds

After:
13528458 function calls (12859134 primitive calls) in 13.917 seconds

The latter optimization makes dump_zone 50-1000 times faster,
depending on the specific question.

BUG=
R=scudette@gmail.com

Review URL: https://codereview.appspot.com/166750044