pgsql: Use binary search instead of brute-force scan in findNamespace()

Use binary search instead of brute-force scan in findNamespace().

The previous coding presented a significant bottleneck when dumping
databases containing many thousands of schemas, since the total time
spent searching would increase roughly as O(N^2) in the number of objects.
Noted by Jeff Janes, though I rewrote his proposed patch to use the
existing findObjectByOid infrastructure.

Since this is a longstanding performance bug, backpatch to all supported
versions.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/73cc7d3b240e1d46b1996382e5735a820f8bc3f7

Modified Files
--------------
src/bin/pg_dump/common.c  |   18 ++++++++++++++++--
src/bin/pg_dump/pg_dump.c |   35 ++++++++++++++---------------------
src/bin/pg_dump/pg_dump.h |    1 +
3 files changed, 31 insertions(+), 23 deletions(-)