Push all DeferredUpdates to POSTSEND queue when running that queue

This avoids putting updates in the PRESEND queue at a point where they
may never get run later in the request. The peculiarity lead to a
regression in 24842cfac.

Move "enqueue" logic to runUpdate() to simplify execute(). If job
insertion batching is strongly desired for a class, then it can use
MergeableUpdate.

Removed unused "update" field in $executeContext.

Bug: T168723
Change-Id: I40d16f6cd0adc8583797b99d859b76a836d362a8