pgsql: bufmgr: fewer calls to BufferDescriptorGetContentLock

bufmgr: fewer calls to BufferDescriptorGetContentLock

We're planning to merge buffer content locks into BufferDesc.state. To reduce
the size of that patch, centralize calls to BufferDescriptorGetContentLock().

The biggest part of the change is in assertions, by introducing
BufferIsLockedByMe[InMode]() (and removing BufferIsExclusiveLocked()). This
seems like an improvement even without aforementioned plans.

Additionally replace some direct calls to LWLockAcquire() with calls to
LockBuffer().

Reviewed-by: Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>
Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3baae90013df58a8d124afa79df07075b8ebea09

Modified Files
--------------
src/backend/access/heap/visibilitymap.c | 3 +-
src/backend/access/transam/xloginsert.c | 3 +-
src/backend/storage/buffer/bufmgr.c | 70 ++++++++++++++++++++++++++-------
src/include/storage/bufmgr.h | 3 +-
4 files changed, 61 insertions(+), 18 deletions(-)