sidekiq-cluster should aggregate STDERR output of sidekiq

Currently sidekiq-cluster ignores the output of the various sidekiq processes. To make debugging potential problems easier this should be changed as follows:

1. Each sidekiq process logs to STDERR
2. sidekiq-cluster periodically reads the STDERR output of each process, then writes it to its own STDERR

mentioned in commit 70a4e8dc

closed via merge request !987 (merged)

mentioned in commit 7e2211bd