pgsql: Install a lookaside cache to speed up repeated lookups of the

Log Message:
-----------
Install a lookaside cache to speed up repeated lookups of the same operator
by short-circuiting schema search path and ambiguous-operator resolution
computations.  Remarkably, this buys as much as 45% speedup of repetitive
simple queries that involve operators that are not an exact match to the
input datatypes.  It should be marginally faster even for exact-match
cases, though I've not had success in proving an improvement in benchmark
tests.  Per report from Guillame Smet and subsequent discussion.

Modified Files:
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    pgsql/src/backend/catalog:
        namespace.c (r1.102 -> r1.103)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/namespace.c?r1=1.102&r2=1.103)
    pgsql/src/backend/parser:
        parse_oper.c (r1.98 -> r1.99)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_oper.c?r1=1.98&r2=1.99)
    pgsql/src/include/catalog:
        namespace.h (r1.51 -> r1.52)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/namespace.h?r1=1.51&r2=1.52)