Record the estimated size of the pack files.

Record the estimated size of the pack files.

The Compacter and Garbage Collector will record the estimated size of
the newly going to be created compact, gc or garbage packs. This
information can be used by the clients to better make a call on how to
actually store the pack based on the approximated expected size.

Added a new protected method DfsObjDatabase.newPack(PackSource
packSource, long estimatedPackSize), so that the clients can override
this method to make use of the estimatedPackSize while creating a new
PackDescription object. The default implementation of this method is
equivalent to
newPack(packSource).setEstimatedPackSize(estimatedPackSize). I didn't
make it abstract because that would force all the existing sub classes
of DfsObjDatabase to implement this method. Due to this default
implementation, the estimatedPackSize is added to DfsPackDescription
using a setter instead of a constructor parameter (even though
constructor parameter would be a better choice as this value is set only
during the object creation).

Change-Id: Iade1122633ea774c2e842178a6a6cbb4a57b598b
Signed-off-by: Thirumala Reddy Mutchukota <thirumala@google.com>