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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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/bd43c50a5bd560224f996d6bd14e1e3cb5a5b06b

Modified Files
--------------
src/bin/pg_dump/common.c | 19 ++++++++++++++++---
src/bin/pg_dump/pg_dump.c | 39 +++++++++++++++++----------------------
src/bin/pg_dump/pg_dump.h | 1 +
3 files changed, 34 insertions(+), 25 deletions(-)