pgsql: Reduce memory usage of tsvector type analyze function.

Reduce memory usage of tsvector type analyze function.

compute_tsvector_stats() detoasted and kept in memory every tsvector value
in the sample, but that can be a lot of memory. The original bug report
described a case using over 10 gigabytes, with statistics target of 10000
(the maximum).

To fix, allocate a separate copy of just the lexemes that we keep around,
and free the detoasted tsvector values as we go. This adds some palloc/pfree
overhead, when you have a lot of distinct lexemes in the sample, but it's
better than running out of memory.

Fixes bug #14654 reported by James C. Reviewed by Tom Lane. Backport to
all supported versions.

Discussion: https://www.postgresql.org/message-id/20170514200602.1451.46797@wrigleys.postgresql.org

Branch
------
REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/209970ded835ffe2f354220da77f3df9a7a7dab4

Modified Files
--------------
src/backend/tsearch/ts_typanalyze.c | 21 +++++++++++++++++----
1 file changed, 17 insertions(+), 4 deletions(-)