pgsql: Fix performance problem when building a lossy tidbitmap.

Fix performance problem when building a lossy tidbitmap.

As pointed out by Sergey Koposov, repeated invocations of tbm_lossify can
make building a large tidbitmap into an O(N^2) operation. To fix, make
sure we remove more than the minimum amount of information per call, and
add a fallback path to behave sanely if we're unable to fit the bitmap
within the requested amount of memory.

This has been wrong since the tidbitmap code was written, so back-patch
to all supported branches.

Branch
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REL8_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6016005118295fdcd97d99a0f372f18a28251c48

Modified Files
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src/backend/nodes/tidbitmap.c | 22 +++++++++++++++++++---
1 files changed, 19 insertions(+), 3 deletions(-)