Made findMissingFiles support scanning files changed in a time range

Change-Id: Ic69249cc3646a04a885ca3b12ea426f1eb2cd171
(cherry picked from commit f2fc6b1f1f902ac4909573e8eab10ce7fa5625e2)
Made findMissingFiles use preloadFileStat() on old versions too to speed it up

Change-Id: I03133c146f5a4f07f27b26dd033d6c93b72aea04
Merge "Made findMissingFiles use preloadFileStat() on old versions too to speed it up" into wmf/1.24wmf18