pgsql: Reduce lock levels of CREATE TRIGGER and some ALTER TABLE, CREATE

Log Message:
-----------
Reduce lock levels of CREATE TRIGGER and some ALTER TABLE, CREATE RULE actions.
Avoid hard-coding lockmode used for many altering DDL commands, allowing easier
future changes of lock levels. Implementation of initial analysis on DDL
sub-commands, so that many lock levels are now at ShareUpdateExclusiveLock or
ShareRowExclusiveLock, allowing certain DDL not to block reads/writes.
First of number of planned changes in this area; additional docs required
when full project complete.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        mvcc.sgml (r2.75 -> r2.76)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/mvcc.sgml?r1=2.75&r2=2.76)
    pgsql/src/backend/catalog:
        pg_shdepend.c (r1.43 -> r1.44)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_shdepend.c?r1=1.43&r2=1.44)
    pgsql/src/backend/commands:
        alter.c (r1.36 -> r1.37)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/alter.c?r1=1.36&r2=1.37)
        tablecmds.c (r1.334 -> r1.335)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.334&r2=1.335)
        trigger.c (r1.262 -> r1.263)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/trigger.c?r1=1.262&r2=1.263)
        typecmds.c (r1.149 -> r1.150)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/typecmds.c?r1=1.149&r2=1.150)
    pgsql/src/backend/parser:
        parse_utilcmd.c (r2.40 -> r2.41)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_utilcmd.c?r1=2.40&r2=2.41)
    pgsql/src/backend/rewrite:
        rewriteDefine.c (r1.141 -> r1.142)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/rewrite/rewriteDefine.c?r1=1.141&r2=1.142)
    pgsql/src/backend/utils/adt:
        ri_triggers.c (r1.119 -> r1.120)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ri_triggers.c?r1=1.119&r2=1.120)
    pgsql/src/include/commands:
        tablecmds.h (r1.46 -> r1.47)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/tablecmds.h?r1=1.46&r2=1.47)