Refactors file watching logic in compiler.

- Fixes the bug where we don't correctly handle moving of the
  directories (because it doesn't generate individual events for
  each file in moved directory).
- No longer watches and process whole source tree, instead respects
  inclusion patterns.
- Removes the extra thread required for each pair of
  ClassPathEntry-PathPrefixSet.
- Stops listening for modification events as we only care about
  creation and deletion in this context.
- Correctly handles overflow by falling back to full-scan only
  for that request instead of all future requests.

With this patch when running Hello sample app in Eclipse with SDM
while using GWT-SDK from source:
 - Uses half of the threads (down from 170+ to 80+)
 - ~15% improvement in first compilation.
 - ~10% improvement in Initial lunch.

Not sure how much of the performance improvements would reflect to
projects with less source roots and simpler source tree.

Change-Id: Ib379e1fa2b6f02405d9fc1a388fa11b8b9a60f83