Introduce PostUploadHook to replace UploadPackLogger

Introduce PostUploadHook to replace UploadPackLogger

UploadPackLogger is incorrectly named--it can be used to trigger any
post upload action, such as GC/compaction. This change introduces
PostUploadHook/PostUploadHookChain to replace
UploadPackLogger/UploadPackLoggerChain and deprecates the latter.

It also introduces PackStatistics as a replacement for
PackWriter.Statistics, since the latter is not public API.
It changes PackWriter to use PackStatistics and reimplements
PackWriter.Statistics to delegate to PackStatistics.

Change-Id: Ic51df1613e471f568ffee25ae67e118425b38986
Signed-off-by: Terry Parker <tparker@google.com>