Remove _REPLACE_PENDING flag from SIM_CHANGED.

This flag is useful if the bcast is a complete snapshot of state
but in multisim it's only half of the story and was causing us
to lose state info when different sim bcasts were close together.

This should be thought about and/or reverted in the future (b/19100123).

bug:18947244
Change-Id: I912274b215d41665a8c179d74aa7f1b897985799