pgsql: Make BackgroundWorkerList doubly-linked

Make BackgroundWorkerList doubly-linked

This allows ForgetBackgroundWorker() and ReportBackgroundWorkerExit()
to take a RegisteredBgWorker pointer as argument, rather than a list
iterator. That feels a little more natural. But more importantly, this
paves the way for more refactoring in the next commit.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/835232c0-a5f7-4f20-b95b-5b56ba57d741@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b43100fa71d7f21751b0b1b9b84833be95336386

Modified Files
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src/backend/postmaster/bgworker.c           | 62 ++++++++++++++---------------
src/backend/postmaster/postmaster.c         | 40 +++++++++----------
src/include/postmaster/bgworker_internals.h | 10 ++---
3 files changed, 54 insertions(+), 58 deletions(-)