pgsql: Change ANALYZE to take ShareUpdateExclusiveLock not

Log Message:
-----------
Change ANALYZE to take ShareUpdateExclusiveLock not AccessShareLock on
the table being analyzed.  This prevents two ANALYZEs from running
concurrently on the same table and possibly suffering concurrent-update
failures while trying to store their results into pg_statistic.  The
downside is that a database-wide ANALYZE executed within a transaction
block will hold ShareUpdateExclusiveLock on many tables simultaneously,
which could lead to concurrency issues or even deadlock against another
such ANALYZE.  However, this seems a corner case of less importance
than getting unexpected errors from a foreground ANALYZE when autovacuum
elects to analyze the same table concurrently.  Per discussion.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        mvcc.sgml (r2.60 -> r2.61)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/mvcc.sgml.diff?r1=2.60&r2=2.61)
    pgsql/src/backend/commands:
        analyze.c (r1.97 -> r1.98)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/analyze.c.diff?r1=1.97&r2=1.98)