Limit sidekiq-cluster concurrency to a maximum of 50 by default

By default, sidekiq-cluster will allocate N threads for N queues specified on the command line, which in turn will require N + 5 Redis connections. Especially when used with the --negate flag, this can lead to using hundreds of Redis connections, which can lead to performance issues.

This change also adds a -m N option to cap the number of threads. For each queue group, the concurrency factor will be set to min(number of queues, N).

Closes #7374 (closed)