pgsql: Fix hstore_to_json_loose's detection of valid JSON number values

Fix hstore_to_json_loose's detection of valid JSON number values.

We expose a function IsValidJsonNumber that internally calls the lexer
for json numbers. That allows us to use the same test everywhere,
instead of inventing a broken test for hstore conversions. The new
function is also used in datum_to_json, replacing the code that is now
moved to the new function.

Backpatch to 9.3 where hstore_to_json_loose was introduced.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f82132f4a15e2a02d984b3ba0ef5b10da1f489bd

Modified Files
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contrib/hstore/hstore_io.c | 43 ++-----------------------------
src/backend/utils/adt/json.c | 57 +++++++++++++++++++++++++++---------------
src/include/utils/jsonapi.h | 7 ++++++
3 files changed, 46 insertions(+), 61 deletions(-)