Multithread symlink forest removal.

This is a second attempt at aosp/2273288 which got rolled back because
it turned out that:

1. We make ~120K symlinks in AOSP (!), all of which need to be deleted
2. System calls are sometimes slow
3. Golang spawns a new OS-level thread for each blocking system calls to
   keep cores busy

All this together means that we sometimes had 10K system calls in
flight, which meant 10K OS-level threads, which is when Go gives up and
says "I created too many threads, please help".

The fix is to move the system calls into a pool of goroutines, which
soon end up on a pool of threads (since they mostly do blocking system
calls)

Test: Presubmits.
Change-Id: Ia9aefff3b0ed373f09bb6c8b2ec1d8b0f00b213b