Cleanup scheduler::buckets class

[clemahieu]

This patch does a number of cleanups to the scheduler::buckets class.

• Extracts a previously nested class into its own file scheduler::bucket.
• Replaces usages of vectors with deques as only indexing is needed, not contiguous allocation.
• Removed buckets::schedule as it's confusing and not needed
• Using more unique_ptr to reduce class definition inclusion requirements