Improve WALRead() to suck data directly from WAL buffers when possible

Applying: Add test module to demonstrate reading from WAL buffers patterns
Using index info to reconstruct a base tree...
M	src/backend/access/transam/xlog.c
M	src/backend/access/transam/xlogreader.c
M	src/include/access/xlog.h
M	src/test/modules/Makefile
M	src/test/modules/meson.build
Falling back to patching base and 3-way merge...
Auto-merging src/test/modules/meson.build
Auto-merging src/test/modules/Makefile
Auto-merging src/include/access/xlog.h
CONFLICT (content): Merge conflict in src/include/access/xlog.h
Auto-merging src/backend/access/transam/xlogreader.c
Auto-merging src/backend/access/transam/xlog.c
Patch failed at 0001 Add test module to demonstrate reading from WAL buffers patterns

error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"