support configurable batches of repositories

[vbehar]

for the moment, octopilot handle all repositories in parallel - creating as many goroutines as there are repositories. we should add a new flag to configure the number of repositories to handle in parallel = the max number of goroutines to create. so that if you have a lot of repositories to update, maybe you'll want to avoid creating too many PR at the same time. to avoid issues such as github rate limiting, or high load on your CI platform, ...