[Core] Speed up job scheduling speed on unmanaged jobs

[Michaelvll]

Although we recently speed up the unmanaged job scheduling speed in #4264, it still takes 2-3 seconds to schedule job one by one. We should speed this up, especially for large-scale job submission, this can cause a significant delay.

Version & Commit info:

• sky -v: PLEASE_FILL_IN
• sky -c: PLEASE_FILL_IN