bufmgr: Optimize & harmonize LockBufHdr(), LWLockWaitListLock()

The main optimization is for LockBufHdr() to delay initializing
SpinDelayStatus, similar to what LWLockWaitListLock already did. The
initialization is sufficiently expensive & buffer header lock acquisitions are
sufficiently frequent, to make it worthwhile to instead have a fastpath (via a
likely() branch) that does not initialize the SpinDelayStatus.

While LWLockWaitListLock() already the aforementioned optimization, it did not
use likely(), and inspection of the assembly shows that this indeed leads to
worse code generation (also observed in a microbenchmark). Fix that by adding
the likely().

While the LockBufHdr() improvement is a small gain on its own, it mainly is
aimed at preventing a regression after a future commit, which requires
additional locking to set hint bits.

While touching both, also make the comments more similar to each other.

Reviewed-by: Heikki Linnakangas <[email protected]>
Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff

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