C_Gather: Set debug #ifdefs to remove set.

C_Gather: Set debug #ifdefs to remove set.

This way when we're confident it works right, we can
remove the set<Context*> and just rely on ref counting.

Further optimizations would include using a spinlock
rather than a mutex, or possibly even just switching
sub_[created|existing]_count to be atomics.

Signed-off-by: Greg Farnum <gregf@hq.newdream.net>