Move PackWriter configuration to PackConfig

Move PackWriter configuration to PackConfig

This refactoring permits applications to configure global per-process
settings for all packing and easily pass it through to per-request
PackWriters, ensuring that the process configuration overrides the
repository specific settings.

For example this might help in a daemon environment where the server
wants to cap the resources used to serve a dynamic upload pack
request, even though the repository's own pack.* settings might be
configured to be more aggressive.  This allows fast but less bandwidth
efficient serving of clients, while still retaining good compression
through a cron managed `git gc`.

Change-Id: I58cc5e01b48924b1a99f79aa96c8150cdfc50846
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>