process: group main thread execution preparation code

[joyeecheung]

This patch groups the preparation of main thread execution into
prepareMainThreadExecution() and removes the cluster IPC
setup in worker thread bootstrap since clusters do not use
worker threads for its implementation and it's unlikely to change.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines