Removed unused test file 'process_spawn.cpp'.

Review: https://reviews.apache.org/r/24582
Added a missing test target in Makefile.am.

Review: https://reviews.apache.org/r/24583
Some Master cleanups.

Review: https://reviews.apache.org/r/24576
Model pending tasks in the Master's metrics and JSON.

Review: https://reviews.apache.org/r/24515
Expose pending tasks during reconciliation.

Review: https://reviews.apache.org/r/24516