Update 404 handling behavior and processing of remote delete change

[chengfred]

Simperium/simperium-protocol@6dd630f updates the behavior for handling a 404, more details in the commit.

Same issue for iOS library: Simperium/simperium-ios#258

[chengfred]

from iOS issue:
Expected Behavior:

1. Ignore remote deletion messages, when the object has local pending changes
2. When error 404 is received, retry posting the full object.

Clarification:
When doing #2 ( retry posting full object (by sending data in d)), remove sv field from change. Reason is: As server has already responded that this object no longer exists (404), specifying sv doesn't make sense anymore.