pgsql: Teach TID store to skip bitmap for small numbers of offsets

Teach TID store to skip bitmap for small numbers of offsets

The header portion of BlocktableEntry has enough padding space for
an array of 3 offsets (1 on 32-bit platforms). Use this space instead
of having a sparse bitmap array. This will take up a constant amount
of space no matter what the offsets are.

Reviewed (in an earlier version) by Masahiko Sawada

Discussion: https://postgr.es/m/CANWCAZYw+_KAaUNruhJfE=h6WgtBKeDG32St8vBJBEY82bGVRQ@mail.gmail.com
Discussion: https://postgr.es/m/CAD21AoBci3Hujzijubomo1tdwH3XtQ9F89cTNQ4bsQijOmqnEw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f35bd9bf359d3d740063997e1cbab9b69df7af99

Modified Files
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src/backend/access/common/tidstore.c | 129 +++++++++++++++------
.../test_tidstore/expected/test_tidstore.out | 28 +++++
.../modules/test_tidstore/sql/test_tidstore.sql | 10 ++
3 files changed, 130 insertions(+), 37 deletions(-)