Reinstate possibility of local process streaming. (#6300)
Reinstate possibility of local process streaming. (#6300)

Previously streams were combined incorrectly, leading to deadlock
whenever a process exit event preceeded end of stream events. We no
longer assume stream shutdown events preceed exit events eliminating
deadlock.

Closes #6089
Fixes #6242
tdyas/pants,twitter/pants,pantsbuild/pants,tdyas/pants,benjyw/pants,wisechengyi/pants,wisechengyi/pants,wisechengyi/pants,wisechengyi/pants,twitter/pants,benjyw/pants,tdyas/pants,tdyas/pants,tdyas/pants,benjyw/pants,twitter/pants,twitter/pants,twitter/pants,jsirois/pants,pantsbuild/pants,pantsbuild/pants,wisechengyi/pants,pantsbuild/pants,tdyas/pants,tdyas/pants,benjyw/pants,jsirois/pants,wisechengyi/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,twitter/pants,twitter/pants,benjyw/pants,tdyas/pants,twitter/pants,pantsbuild/pants,wisechengyi/pants,wisechengyi/pants,tdyas/pants,pantsbuild/pants,jsirois/pants,twitter/pants,wisechengyi/pants