pgsql: Don't set ThisTimeLineID in checkpointer & bgwriter during recov

Don't set ThisTimeLineID in checkpointer & bgwriter during recovery.

We used to set it to the current recovery target timeline, but the recovery
target timeline can change during recovery, leaving ThisTimeLineID at an
old value. That seems worse than always leaving it at zero to begin with.

AFAICS there was no good reason to set it in the first place. ThisTimeLineID
is not needed in checkpointer or bgwriter process, until it's time to write
the end-of-recovery checkpoint, and at that point ThisTimeLineID is updated
anyway.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/1a11d4609efaae39d9b7472fb965bca1c0aeda01

Modified Files
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src/backend/access/transam/xlog.c | 7 ++++++-
src/backend/postmaster/bgwriter.c | 6 ------
src/backend/postmaster/checkpointer.c | 6 ------
3 files changed, 6 insertions(+), 13 deletions(-)