Estimate the amount of memory used by a PackWriter

Estimate the amount of memory used by a PackWriter

Memory usage is dominated by three terms:
 - The maximum memory allocated to each delta window.
 - The maximum size of a single file held in memory during delta search.
 - ObjectToPack instances owned by the writer.

For the first two terms, rather than doing complex instrumentation of
the DeltaWindows, we just overestimate based on the config parameters
(though we may underestimate if the maximum size is not set).

For the ObjectToPack instances, we do some rough byte accounting of the
underlying Java object representation.

Change-Id: I23fe3cf9d260a91f1aeb6ea22d75af8ddb9b1939