Allow PackWriter callers to manage the thread pool

Allow PackWriter callers to manage the thread pool

By permitting the caller of PackWriter to select the Executor it
uses for task execution, we give the caller the ability to manage
the lifecycle of the thread pool, including reusing it across
concurrent pack generators.

This is the first step to supporting application thread pools
within Daemon or another managed service like Gerrit Code Review.

Change-Id: I96bee7b9c30ff9885f2bd261d0b6daaac713b5a4
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>