Spawn grouping of fingerprints with free semaphore.

The previous implementation spawned N goroutines to group samples
together and would not start work until the semaphore unblocked.
While this didn't leak, it polluted the scheduling space.  Thusly,
the routine only starts after a semaphore has been acquired.
Merge pull request #262 from prometheus/feature/storage/fix-grouping

Spawn grouping of fingerprints with free semaphore.