pgsql: Put back a backwards-compatible version of sampling support func

Put back a backwards-compatible version of sampling support functions.

Commit 83e176ec18d2a91dbea1d0d1bd94c38dc47cd77c removed the longstanding
support functions for block sampling without any consideration of the
impact this would have on third-party FDWs. The new API is not notably
more functional for FDWs than the old, so forcing them to change doesn't
seem like a good thing. We can provide the old API as a wrapper (more
or less) around the new one for a minimal amount of extra code.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/4db485e75b9672126963ae4052b50f473b30a097

Modified Files
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src/backend/utils/misc/sampling.c | 46 ++++++++++++++++++++++++++++++++++++-
src/include/commands/vacuum.h | 5 ++++
src/include/utils/sampling.h | 16 ++++++++++---
3 files changed, 63 insertions(+), 4 deletions(-)