Fix a race in extent_grow_retained().

Set extent as active prior to registration so that other threads can't
modify it in the absence of locking.

This regression was introduced by
d27f29b4 (Disentangle arena and extent
locking.), via non-obvious means.  Removal of extents_mtx protection
during extent_grow_retained() execution opened up the race, but in the
presence of that locking, the code was safe.

This resolves #599.