pgsql: Move bgworker specific logic to bgworker.c

Move bgworker specific logic to bgworker.c

For clarity, we've been slowly moving functions that are not called
from the postmaster process out of postmaster.c.

Author: Xing Guo <higuoxing@gmail.com>
Discussion:
https://www.postgresql.org/message-id/CACpMh%2BDBHVT4xPGimzvex%3DwMdMLQEu9PYhT%2BkwwD2x2nu9dU_Q%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4d22173ec08cfdb83f61cfff1d6fa67fe7d83296

Modified Files
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src/backend/postmaster/bgworker.c   | 83 +++++++++++++++++++++++++++++++++++++
src/backend/postmaster/postmaster.c | 83 -------------------------------------
2 files changed, 83 insertions(+), 83 deletions(-)