pgsql: Expose oldSnapshotControl definition via new header.

Expose oldSnapshotControl definition via new header.

This makes it possible for code outside snapmgr.c to examine the
contents of this data structure. This commit does not add any code
which actually does so; a subsequent commit will make that change.

Patch by me, reviewed by Thomas Munro, Dilip Kumar, Hamid Akhtar.

Discussion: http://postgr.es/m/CA+TgmoY=aqf0zjTD+3dUWYkgMiNDegDLFjo+6ze=Wtpik+3XqA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f5ea92e8d620a1260d3427b92fa350bbf36594a2

Modified Files
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src/backend/utils/time/snapmgr.c | 55 ++---------------------------
src/include/utils/old_snapshot.h | 75 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 77 insertions(+), 53 deletions(-)