Pass PackConfig down to PackWriter when packing

When we are creating a pack the higher level application should be able
to override the PackConfig used, allowing it to control the number of
threads used or how much memory is allocated per writer.

Change-Id: I47795987bb0d161d3642082acc2f617d7cb28d8c
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
9 files changed