bookmarks: directly use base dict 'setitem'
The bmstore '__setitem__' method is setting an extra flag that is not needed
during initialization. Skipping the method will allow further cleanup and yield
some speedup as a side effect.
Before:
! wall 0.009120 comb 0.010000 user 0.010000 sys 0.000000 (best of 312)
After:
! wall 0.007874 comb 0.010000 user 0.010000 sys 0.000000 (best of 360)
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