Reverts 6657d06f793a3bd1523f8daf8ce9db5f5485b5c4.  The reference count in
refcnt mode DOES need to be set to zero, although it's not clear why.

Adds a new configuration for cross-checked gc (xgc) with optimizations.  Fixes
a bug in (xgc) that caused it to fail (LockedPtr should take a NodePtr const &,
not a Node *).

Running on yellow-eyed.
------ Config refcnt-opt ------
reverse              0:01.12
tak                  0:00.65
rfib                 0:00.15
exp3_8               0:00.21
tree_insert          0:00.43
tree_insert_32       0:00.43
------ Config gc-opt ------
reverse              0:00.69
tak                  0:00.61
rfib                 0:00.12
exp3_8               0:00.10
tree_insert          0:00.52
tree_insert_32       0:00.53